What makes satan evil?

He punishes those who are evil for eternity but is still the bad guy id think hes more of a jail warden kind of thing keeping bad people in hell but not nessesarily completely evil at least in this aspect he is a nessesary part of the christian universe what would happen if he one day decided to go upstairs to heaven and say sorry and he turned good that would throw the christian world off balance no? This is a serious question to christians please try to answer this since to my knowledge there is no "read this if the devil turns good" section of the bible but its a what if kind of question fyi im an atheist but im curious so please dont answer this angrily just because an atheist asked it