When someone cuts their arms, does that mean they have a mental illness?

Not necessarily. Not if they're just cutting themselves just for kicks, then they might be just sadistic lol. But otherwise yes, they have some form of mental problem, and it depends on the reason for cutting what the specific mental problem is. A mental problem is something messed up in someones brain which people think isn't normal just to clarify that. People don't "Just cut cause there depressed". What kind of fu*ked up logic is that????