Honestly? I guess my limit would depend on the context. Two examples that come to mind.
About 4 years ago I teamed up with this cat from another country (not going to give details just in case) to fuck people over on an online RPG out of virtual items, like gold and shit (dont laugh we had $400 days back then), and after getting in tight with him I found out he was pretty heavy into moving dope (coke, E, weed). I happened to know a lot of dealers at the time, and he wanted to arrange a deal. Then found out these guys had access to handguns which sell for a premium in his country since they are banned. Long story short I cut ties with the guy around that time. It was hard since I was pretty much going to be making large sums of cash for doing nothing, but it definitely was a line I was not willing to cross.
Few months later guy hit me up randomly and asked me to do something very shady online. Essentially jacking people's credit cards and getting a % of whatever was jacked.
I turned that one down as well. I knew enough to probably make 5k a day at the time doing this, and was pretty hard up financially, so guess I am proud I stayed above that and stuck to things that were legal (shady maybe, but legal).
I guess I kinda new the people who would get the handguns, and they probably would end up with some bodies on em. Won't say it didn't tempt me, but if there is a line for me it would be before those things.