I'm going to take my roommates cat
Thats not okay. Shouldn't you get permission before you take their cat?
I love her, my roommates doesn't deserve her, he is so unhappy with this cat, even the cat doesn't want to stay with him
Sure, maybe your roommate doesn't treat the cat well but that doesn't mean you have the right to steal his pet? It's not good to steal and you will be caught fairly easily. There are ramifications you need to consider before you take his cat.
Okay fine  I'll leave it.
It's good that you're going to leave it. Just because you believe they don't want the pet doesn't make it true. If you really want to take care of it ask them and maybe they'll agree to it.