I'm going to leave/ghost my gf that was financially dependent on me without warning after discovering she cheated on me
Why do you think ghosting her is a good idea?
I found out about her dating other guys during my black days
I'm sorry your girlfriend cheated on you. It is devastating to find out you've been deceived; you are well within your rights  to kick her out. I would suggest handling the breakup as amicably as possible so you don't get dragged into fights and can break up cleanly.
Why am I feeling like a bad person? I feel like I'm doing the right thing since she's been talking to other guys.
It can be hard to do things like that to the people we cared for, especially when we feel betrayed by them.  These feelings show you're a fundamentally good person and she took advantage of that.  You need to protect yourself from people like that.