Yeah, it's a very weird moral argument. I want to say, one based around idealism, if I'm recalling my terms correctly (maybe some individualism in there too). The general argument of it being that evil acts come from inner evil, not from outer circumstances, so doing something that is under the umbrella of "bad stuff" has a "corrupting" influence and will "make you evil" like some sort of Evil Meter that fills up each time you do a "bad thing." (As much as I like them as games, video games like KOTOR sort of do this literally.)

This position also seems to treat "bad stuff" as all corruptive and all on a sliding scale. So like, petty thievery would probably be on the lower end, but might "corrupt you" into "darker stuff" like physical violence.

I think the only truth in that conception of it is that if you become desensitized to certain acts, you might be more apt to do them again without the normal mechanisms of shame, guilt, traumatic reaction stopping you. But as we know from history and present, sometimes people go through with horrific acts in spite of there being collateral damage in the form of them having a traumatic reaction to doing it. Because the external processes and pressures supersede the internal "striving." Which is a fancy of saying, "The world is not defined solely by personal willpower. Fuck you, rugged individualism."