Quite. Tsath' was almost completely unarmed, bar a small folding penknife, he didn't have any of his guns with him because he was in town (and just try concealing a dart or gauss rifle in the first place...not exactly the kind of hardware that lends themselves to being tucked in a coat pocket

Had he tried to pull the knife, then most likely the two wouldbe muggers would have seen it and jumped him the moment they did so. Weather or not toady could have drawn the blade is anyone's guess. Failure however would probably have led to HIM getting stabbed with his own blade.
And in any case, he sees absolutely no difference between using a concealed firearm were he in a situation that it were a possible route of defense, and using poison. The end is identical, the only difference is the means by which that end is achieved.
And as for the thug in jail, Tsath' had done NOTHING to initiate hostilities, he had kept himself to himself, spent most of his time reading, listening to music and when he could get any, smoking a bit of H or doing subutex. He'd never even SPOKEN to this fucking arsehole, and there simply wasn't any way he could have beat the guy one on one, he'd only just recovered from forced cold-turkey withdrawal from barbital and chlormethiazole, plus opioids. He'd very, very nearly starved to death during the DTs. So he had a pretty stark choice. Either be sexually assaulted, or terminate the threat by any means necessary.
So he overcame the severe physical deconditioning, and used what remained available to him, namely his intellect. That and biological/chemical warfare.
And he did not poison anybody to 'benefit' himself. He did so to save himself. There is a big difference. The reason was not personal gain, but continuing personal safety when extirpating vermin was the necessary course of action. Tsath's fucking blowed if he's going to get buggered or cut up.
He is generally a peaceful, thoughtful and kind individual, but anybody preparing to hurt him, or hurt his loved ones, they better bee prepared to pay a high
price for failure. Because to aggressors he is as cold, calculating and vicious as he is kind to those he cares for. For his friends, he will go to the ends of the earth. His enemies by the same token he will chase right up to the bank of the Styx.