Though the immunity thing actually probably accounts for at least as many (and probably more) kills than it has ever cost him:

1- If characters are in there, that probably means they are currently vulnerable to the poisons.
2- It's a dangerous enough place/fight on its own that being surprise attacked in the process of trying for immunity, against a character you don't feel comfortable fighting without immunity even under neutral circumstances (hence why you are there trying to get immunity in the first place), can easily turn into a real cluster-f for the victim.
3- Having a location that is a magnet for your enemies is the perfect complement to trapper skills. Outside of their usefulness in raids, I'd imagine the hardest thing about traps is getting people to walk into them.

If the Imm's are able to track pk's by area, I'd bet that over this thief's lifetime this area would be up there with cabals/galadon for most pk's in the mud. And the better/scarier he gets, the more the immunity draws his enemies right to this place for him, like moth's to a flame.

That aside, if the immunity can be acquired, it does have to be a kick in the junk for poisoners, and especially so against ones who might not have village/defender/trapper skills making them a lot tougher.

All that said though, I agree the poisons are unbalanced. The knock-out is so successful that there's just very few characters where the risk/reward is ever in favor of fighting him without immunity.

Warriors with entwine?
Duergar warriors w/Avalanche?
In form tigers?
Maybe conjurers? (I admit I don't know much about them)

Anything else probably has limited odds to ever make him pay for going after the (highly) successful knock-out, and once you're knocked out, losing inventory, 10hrs for spells to fall, and failing a good percentage of anything you try via mind control effects (which only even applies if you've saved against the forget that tick) that is enough combined to put the majority of characters at significant risk of dying. And we haven't even considered neuro poison landing after the fight starts, or that for this particular character, there's a bunch of traps waiting when you flee.

So like people have said, the problem is that there's still a low but real risk of dying even for the builds best equipped to survive him, a fairly high risk of dying for everyone else, and not many builds at all that have a better chance of killing him than he does of killing them.

I personally think it'd be more balanced to reduce knock-out chances significantly, and cut the duration of all the poisons down to around 5hr KO, 5hr MC, 5hr Neu, and then find a way to make it so that those changes don't force thieves to burn through preps and have to spend all their time prep gathering. They'd still be as deadly as ever for 5hrs and they'd still mess people up in drawn out raids since the thief can just engage/poison again, but people wouldn't be stuck resting off 20+hr effects, and with more engagements there is at least the sense that you have more chances to turn the tables on him.

So maybe like someone said, 3-4 uses from a poison ingredient instead of one, and then have durations cut down to ~5hrs. This character has a lot more going for him than a regular poisoner (trapper, village resist, defender) but I really think that would make things more balanced against both him and regular poisoners too, and reduce the amount of "immunity or I'm avoiding him at all costs" class dynamic.