I have been there done that. Having a negative attitude and being bitter is grounds for termination.
Even if you try to suck it up your language, mannerisms, and reactions to stresses show someone no one wants to do business with. Guess who then gets the poor performance evaluation or let go? YOU.
Part of being competent at your job is loving your company and what you do. It is part of integrity. Yes, I have experienced bad things both sides from me my employer to myself reacting angrily. At this stage I update my resume and leave. I figured if I do not do it now it will be made for me by HR firing me and now lowering my value as I have to explain in my next job interview why I have hole and went happened at my last employer. Boy, you do not EVER want that! It is a kiss of death.
I have never wanted to do such things as keeping passwords or screwing any employer. Even the ones I left I was angry at management, but did not want revenge. If I ever get to that stage it is sickening to myself, employer, and coworkers and nobody wins in such a situation.