I actually have 3 questions, but as the subject says, the main one is that does a kill-free playthrough of Deus Ex have any benefit other than personal satisfaction?
I'm basically near the beginning, but so far the only person I've killed is Agent Navarra(sp), and she forced my hand.

But now my brother is in trouble, and he is all "Go! I'll stall them!", but I'll have none of that sacrifice BS, so I stuck around to help him out, but I don't think it is possible without killing the guys after him. Only after killing one or two he tells me "Go! I'll hitch a ride to Hong Kong and meet you there!" so maybe he didn't need my help after all? Should I just leg it? Will he hate me for that?

I know that it may seem cheaty to ask such help from such a choice-driven game, and the best answer would be "Do what you feel is right!", but if it has no benefit, and makes the game almost impossibly hard, I'd rather not.

Finally, what the hell is up with tranq darts? Are they as useless as they seem? I fire one from as close as possible and leg it to a hidey-hole, but the guy I hit runs around screaming and alerting everyone around for half a minute before dropping unconscious. Is there some more immediate long-distance non-lethal incapacitator?

The game hammers in a completely ridiculous sense of morality in the first two missions. Armed terrorists all over the place that will try and kill you on sight? Well remember that a good police officer would NEVER use lethal force against anyone for any reason, not even armed criminals with murderous intention. The proper way to achieve mission objectives does not take into account the safety of law enforcement officers, they are expected to go about the most difficult way to incapacitate ARMED TERRORISTS without compromising mission objectives. Yeah, seems like a plausible dystopian future...

After the first two missions the game hardly ever cares to make a distinction between, dead, unconcious and unaffected NPCs. The story is not affected by your minimal force choices so if you are not enjoying the playstyle you should change it.

The upside to playing it without killing people is that it is ultimately a stealthy playstyle, which usually means more exploration.

To be fair, the people harping on about that have their own reasons for doing so. Orders from upstairs are to shoot on sight without even attempting arrest. On their own, they aren't that good, but there is a way to increase their effectiveness dramatically. Chuck a gas grenade near your target, then hit them with a tranq dart; the gas immobilizes them while the tranq goes to work. It's especially effective if you can target a group with a gas grenade.

Well, the simple (and spoiler-free) answer to your second question is: ultimately, no, it doesn't matter if you stay and fight with Paul or just take off.

The more detailed answer requires slight spoilers. Nothing plot-affecting, but still spoilerish. Basically, if you don't stay and kill everybody for him, he'll die, and even if you do stay, there's a chance he'll die anyway, since the AI really isn't that good at fighting other AI. This will slightly change a few minor details about some of the following missions, but like I said previously, it ultimately doesn't have much of an affect. If he lives, you'll have to get something from him in the following mission. You will then be able to talk to him in person a little later in the game. Other than that, he communicates with you a few times via infolink and, near the end, via one of those holographic communicators. However, the only think that is significantly different about him dying is that you won't get to talk with him in person. The thing you have to get from him must be retrieved from his body, and the later infolink and holographic communications still occur, just with a different character (and thus slight variations in what's said, though the gist of the communications remains the same).

I may be remembering incorrectly, of course, so someone else feel free to correct me if I'm mistaken.

Just don't leave through the window, then. It makes the game assume you left him to his fate.

Actually if I remember correctly (it's entirely possible that I do not) isn't it the case that as long as the tranqued guy isn't in eyeshot of the other guards, he can scream all he wants? I usually tranque them just as they finish a conversation and they're walking away from each other.

Anyway, it's better to follow their pathing and get them in a quiet spot, there's generally always one it you are patient.

However, yes. My playthoughs usually end up as a run and gun. No Kill/Stealth can be fun and novel for a while but it's such a long game for one to be so constantly diligent and frankly it can bore the pants off you.

For the best effect for tranq darts, aim at the head. It's an instant knock out that way, even on higher difficulty settings. Likewise, there's a baton that's very easy to get in the first mission. Carefully sneak up behind people and again, hit them in the head. No need to use a stun baton or gas grenades at all if you have some patience.

Headshots are not instant knock outs, and the baton only really works against unaware targets struck across the back. Aiming for the head is not as easy as it looks and it is not worth the risk when simply striking them anywhere on the back torso does somewhere around 60-70 damage.

Still, tranquilizer darts should be aimed at the head when possible if you are going to use them at all and the baton can be a perfect replacement for the prod for stealthy players.

I've done the stealthy non-lethal playthrough so many times though. Nowadays I walk up all determined and cool to zap people in the face with the prod, or toss a gas grenade at groups of enemies and walk over to them (holding back the tears, of course) and execute them in cold blood. You know, for great justice in this post-911 era of blockbuster cinema glorifying all things military, extra-judicial killings and surveillance.

Unlike 3 you don't really get rewarded for non-lethal, although in early stages it can change some of the cutscene dialog, so it'd only be for personal satisfaction.

It gets really bloody hard later too, as the only stealthy non-lethal is the Prod, so you end up having to go super stealthy and even taking some of the otherwise more useless skills just to give you ways around stuff.

Everything else will trigger an alert to anyone in earshot, including gas grenades and the utterly stupid tranq crossbow (I was so glad the tranq in DX3 fixed this!).

I played it stealthy non-lethal until I got maxed rifles, then it was just stealthy >:P

While DX3 made the tranq more powerful, I wouldn't necessarily consider it better. In DX, non-lethal is a decision by the player to take the moral route even though it's more difficult. In DX3, non-lethal is just another choice. They made the crossbow better than the lethal weapons except for the scarce ammo.

As for the OP's questions...

***SPOILERS***

Non-lethal has very little effect on the game environment beyond the first two missions.

It's possible to beat the game without killing anyone, directly or indirectly. Even Navarre. You have to exploit a few quirks in the AI. Well, without killing anyone except for the time you launch that nuke. Or scuttle that super freighter. And I'm pretty sure the first terrorist leader, Lebedev and that shifty mechanic don't get to live very long. But besides all the deaths JC is responsible for, you can beat the game without killing anyone. Now it kind of makes sense why nobody calls JC a saint.

Paul only dies if you leave through the window. He's immortal and can easily clean out the hotel on his own until he's out of your sight and magically teleports away. This means it's possible to save Paul without killing anyone directly. So much for that injury. Maybe he was faking it because he wanted to save you.

Unfortunately, I don't think it's possible to save him without anyone dying since he'll kill the group who storm through the door even if you leg it for the exit. You can minimise casualties by popping cloak/camo and running for the exit. They didn't design the story for this eventuality but I'm sure you can think of something.

One of the fun tricks that I learned from an anti-walkthrough on it-he.org is that you CAN save Lebedev. Sort of. Stun him and carry him with you. As long as you are carrying the body when the level changes, you'll bring him with you. I did as suggested by the anti-walkthrough and left him in safer hands in China.

You can exploit this trick to do other fun things. Also inspired by it-he, I may have carried Simons with me from the ocean level to the missile base and arranged for him to take a trip to the stratosphere....

0_o Is it possible to post a screenshot? I HAVE to see this with my own eyes!

ps: Don't all augmented agents explode when their life is really low? Or if you knock them unconsious?

I did this playthrough a couple of years ago, so I'm not sure if I still have the save games. I'll come back and post if I can find them! For Simons, I set his body on top of the missile silo doors at the surface, went downstairs and triggered the missile, and then made my way back upstairs in time to see his body on top of the missile as it left the silo.

As far as Simons exploding, I'm quite sure I carried him around without hit detonating, but I suppose that could have been a strange bug. Maybe I should try another playthrough and see what I can do...

EDIT: Sadly, it looks like my save games are long gone. It'll take another playthrough to try it again.

No problem. I will just play it again and find it out myself!

I have my old saves in my backup disk and I will try to make it kill-free. Trying not to kill anyone and in the "realistic" difficulty is the best way to enjoy the game.