perhaps you've heard that two wrongs don't make a right?
if a company screws you over, you don't try to screw them back. that makes you just as bad as they are. the right thing to do is walk away, think about the warning signs that were almost certainly there from day one, and don't get into a situation like that again.
I'm not arguing that anyone should be screwing anyone over. Taking revenge for the sake of revenge is a monumental waste of time and energy, and can lead to disastrous career and legal consequences.
In the situation I outlined-- of a person being outsourced, and told to train his cheap-hire replacements before being let go-- the reason he does a poor job of training them is not revenge. It's not an emotional decision, but a rational one: he wants leverage for the negotiation of severance and references, e.g. "Sure, I'll train my replacement, in exchange for [X]." It's neither ethical nor unethical; just business the way it's been done for centuries.
if a company screws you over, you don't try to screw them back. that makes you just as bad as they are. the right thing to do is walk away, think about the warning signs that were almost certainly there from day one, and don't get into a situation like that again.