When a main character's dad dies in a book. Can the main character become a whole new person (bad way)?

Yes, actually. Because he/she could be totally tramatized and just, lose control of everything. I've writin a story (not published) but it did have someone who died then someone just lost control. Hope this helps. :)

Depends on the situation.

In my first novel, the main character lost his father when someone caused him to be wrongly convicted of the President's murder, therefore leading onto angry mobs beating him to death and a police shootout.

The main character, 5 years later, now 19 years old, vows to take vengeance against the man who set up the murder.

I suppose the main character kind of turned bad.

These sorts of traumatic experiences greatly shape who we are. It would be very realistic for this to change your character, but not entirely.