Rattus Rattus is a Rattkin (I think part of the Rattkin Razuka faction), that walks the Arnika-Trynton Road. He gives you a broken quest, to cash a check for him at Arnika Bank. What he really means is to rob the bank. He gives you a holdup note and a Zip Gun and ammo, and says he'll meet you in Trynton. He is never seen again after he wanders from your line of sight.
Quest-wise, it makes sense to kill him, as he sets you up for failure if you actually read the note. It will probably make Milano Calzone and Don Barlone hostile on sight. I believe that Milano is a pushover, but the Don is level 20, so I probably won't go to the 6th Bough until a lot of grinding has been done.
The Swamp is a great place to level grind, Oozites give lots of experience, the various insects aren't too tough, and neither are the slimes. Watch out for the crocodiles, they hit hard, and avoid the mares at all costs. I find them extremely annoying. They run away sometimes, and can Turncoat your party.
I always kill Don Barlone, Milano Calzone, and Crock anyways (they're all shady characters, and I lose no faction rating with anyone else). Killing Rattus Rattus will give closure, in that I know what happened to him ;)

yes killing him is one option and doing his 'quest' is another or you can just take the free gun and walk away as the 3rd option... each to their own I guess

Otherwise, you really don't know what happened to him, right? At least if he dies by my hand, there will be "completion".

I've never heard of that. If it is possible, it might be a very low percentage chance. Most websites say that it's a broken quest, and he is never seen again.

he does wandering around but I've never seen him on a different map... was this game using a mod?

I'm going by the official guide. I guess he never returns. This would mean that killing him would be the only way to be sure of an outcome.

I didn't use any mods back then. It was the vanilla game patched to 1.2.4. The playthrough was in 2003 or so, so it's been a while. Since it was my first playthrough I also didn't know the maps well, and there was lots of aimlessly wandering around. For example, I missed the information where the Rapax Camp was, and was walking 2-3 times through various maps hoping to find the Camp in some corner (not realizing that it was its own map). It must have been then, where I stumbled upon Rattus again.
Of course there is always the possibility that I mixed something up in my memory. Yet I'm sure I met him a second time after doing the bank job.

I do remember many years ago reading that Rattus Rattus will show up again, maybe I read it on the ironworks forums. I must admit in all my playthroughs I just take what he offers and then ignore him and his quest.
I have never actually seen him again.

If you ask Milano Calzone about him afterwards he says something along the lines of "he took a bullet in Arnika." I don't think it's a broken quest, exactly. In my opinion it's intended to be unfinishable. It's a gag to introduce how silly and untrustworthy the Rattkin are.

Which leads me to another question, if anybody knows (I've never whacked Rattus before):
Do you HAVE to visit the Swamp first, and have the Savant freak out at you, and drop the fake AD, before visiting Don Barlone, for him to have the real AD? I wonder if I can softlock the game if I kill the Don too early, if he's hostile on sight, before the Savant loses the AD, if anybody knows what I'm trying to say.

from memory the trigger is having one of the 3 items so if you happen to visit after getting the mook item but not talking to dark S then it should still trigger

Thanks. Maybe visiting the Swamp first after getting the +5 to Intelligence Fountain should be the way to go. Just to make sure that the Rattkin stole the Astral Dominae.

There are a number of things that trigger based off the number of artifacts you've collected, like the AP landslide or the various faction attack groups in various zones. However, the trigger for Barlone to start selling the real AD is in fact the gas vents in the swamp. He will forever just say "we ain't got it yet" until you've trigger the Savant appearance in the swamp.

It's a completely bizarre condition, and I have no idea why they chose that instead of just having it as a fixed location appearance, but so it goes. You can still get the true AD off Barlone's corpse at any time, though. Even if he says they don't have it yet, or you never even spoke to him in the first place, he will drop it when he dies. Killing him will also trigger the Savant to appear in the gas vents, so you can still retrieve the CM without bloodshed at that point.

Thank you. That was kind of what I was hoping for: A true continuation of the story. To be sure though, at least from a gameplay perspective, it would make more sense for me to stumble upon the Savant in the Swamp, have him throw his temper-tantrum, drop the fake AD, then visit the Don and kill him on sight (he's mad because I killed Rattus), and take his real AD from his corpse, without having to pay for it.