Cursory google fu reveals that CS is ordinarily not lethal. However, one guy did die when police sprayed him and held him down, though it's suspected that it was suffocation from being held down, and not the gas, that killed him.
Of course, according to RAW, all stun damage can kill you. That makes sense for narcoject -- is an anesthetic, and all anesthetics can kill you in an overdose. But getting a quadruple dose of tear gas shouldn't do it... let alone pepper punch! Pepper punch could kill you even faster because it does more stun damage, and that's just not right... I've never heard of someone dying from eating chili peppers, unless maybe they were allergic.
The best explanation I could think of for those being lethal would be that somehow, you can't breathe while being doused by them, and you suffocate. It's not that plausible, but the other option is to shrug your shoulders and say 'meh.'
Either that, or house rule that some stun damage can't overflow, like damage from non-lethal gasses.