You have bigger enemies more worthy of that personal risk. This comment bewilders me a bit.

Don't commit felonies because you're unhappy with your former employer.

Is it a felony to crash someone's computer?

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

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

A deliberate act of revenge against a former employer... wouldn't be given much benefit of the doubt by the courts.

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.

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

Possibly, yes.

If done deliberately…

Nothing wrong with getting some satisfaction. Just don't do it in a way that can be traced back to you.