I just beat the game as an evil character doing the evil quests. I joined the Khans, killed Killian for Gizmo, joined the Skulz, did Decker's quests, and killed the Blades for Zimmerman. I took the infiltration route to destroy the Cathedral and Military Base. Even in the end, I still ended up with a reputation of five, not enough to kill the overseer.

I held off on collecting negative reputation points until I had Power Armor and had access to the majority of the merchants. I got the water chip but did not repair the Necropolis pump. I killed hostile surface ghouls in Necropolis and earned the Champion reputation title.

Overall, I found that it is impossible to be evil in this game, unless you go on a killing spree, slaughtering entire towns. Then again, I guess this applies to pretty much every game in the franchise, except maybe Fallout 3.

Agreed, the impossibility to play a fully evil game carries to the three first titles of the early franchise, at least.

In Fo1 and Fo2, the guides indicate that you can do a lot more evil than you describe you did, which makes the player's life very much harder, but finishing both games requires destroying the Enclave at Mariposa Military Base, The Cathedral and the Enclave Oil Rig (if not Navarro, I forgot whether that's mandatory): it's impossible to side with the Enclave.

In Fo3, the main story line makes it possible for the player to insert the modified FEV into the Purifier (game over), or in the Broken Steel add-on, target the Citadel instead of the Enclave platform. However, there's no way to side with the Fo3 Enclave to the extent that siding with the Legion in FoNV is possible; in FoNV, the player can make the Legion win, defeating the NCR & BoS in the process, targeting the NCR in Lonesome Road, etc.

Not sure yet about Fallout 4 and what being really evil would entail... maybe siding with the Institute, destroying all of the Minutemen, the Railroad and the BoS, becoming the Nuka-World Overboss, slaughtering innocent civilians everywhere as usual, and I don't know what other mischief ?

You don't fight the Enclave in Fallout 1, just the Master and his mutants.

And no, wiping out Navarro isn't required either.

In my recent playthrough, I didn't kill any children, so I didn't have the Childkiller title.

The only other evil acts that I can think of in that game is massacring entire towns and killing everyone, except the Vault 13 Overseer, but that's for true psychopaths only in my opinion, and maybe low-intelligence characters..

Probably I'm wrong, but killing the overseer is not due a perk ? I mean, at my last play I'm pretty sure I was "good". Or at least in comparison to your character xD . And yet, the overseer was left in pieces when my character was exiled

Were you a Berserker or Childkiller? Did you take Bloody Mess?

I only took the bloody mess.

I was playing a nice person. I even did not went for the "All sex" perk (or whatever is called when you become a porn star)

P.s. sorry for the delay, the notifications at GOG forums normally doesn't work for me :(

You don't become a porn star in Fallout 1. That's 2. Bloody Mess automatically makes you kill the overseer at the end of the first game regardless of how you played, even as a good character.

I've also been told that randomly slaughtering towns for absolutely no reason, other than to kill everything is pointless, stupid, and boring.

For some reason, killing the residents of Junktown reduces your reputation more more than other settlements, even if you only kill quest-revelant characters, such as Neil, Trish, Killian, and a few guards. It's practically impossible to become a berserker unless you go on a murder rampage.

Are you playing with mods, or is this vanilla? (ie FIXT)

Vanilla. Just the semi-official patches by TeamX.