Do you think it is okay that I'm not going to let anyone know my roommate left her cat alone over the break?
That depends. Is there some reason why you're not telling anyone about the abandoned cat?
I want to let her cat die of starvation over the break.
It is not right to avoid telling others when a pet is left alone just because you wish malice towards the pet. Inform someone about the cat so that they can go and feed it.
It is not like I hate him. I sometimes gave him extra food.
That you were once nice to the cat doesn't make a difference if you let him die now. If you really don't hate him, you should tell somebody right away that the cat was left alone.