If it causes more than $5k in damage. Otherwise, it's a misdemeanor.

But you probably don't want to be investigated for either.

Would it even crash a computer? They would fill up their hard drive but that would just yield warnings to the user in most operating systems. Chances are they would kill it manually because it would take a long time

Violations of the Computer Fraud and Abuse Act (CFAA) can be either misdemeanors or felonies. It's definitely broad enough that doing so could get you in serious trouble if pursued.

Possibly, yes.

If done deliberately…