I have this crime boss villain. He's charismatic as he is ruthless, and he knows when to call quits when the situation goes south. If, for some reason, he got possessed an ancient soul of a warrior who's been fighting evil for generations, would that ruin the character?
Don't know. You're putting the cart before the horse. Writing pure evil as the antagonist is just lazy writing. Ask yourself the following questions.
1. Who is this crime boss?
2. How did he grow up?
3. Why did he become the boss?
4. What was his motivation to doing the things he did?
5. Why would people follow him?
6. What did he do to get people to follow him?
7. What qualifications did he have to become the boss?
8. What is the reason he's not offed by ambitious underlings or rival gangs yet?
9. If everything ever goes south, can he show that he is the boss through his presence and actions?
10. Does he has a family? What is his family's role in all this?
Now, if you make him get possessed by a righteous warrior's soul, you should ask yourself.
1. Who is this righteous warrior?
2. How or why did he become righteous?
3. What made him the man he is?
4. How would he react if something happens in from of him that goes against his philosophy and morals?
5. How would he feel about fucking someone else's wife? Yes, it's the crime boss's wife, and he's inside him, but she's technically a stranger.
6. How would he feel about fucking someone else's mistresses? Every crime boss has a few mistresses, it's just how it is, sex is power.
7. How would he feel about acting like a dad in front of the children he do not know? What if the daughter has a crush on him?
8. How would he feel about the crime boss's little girl stripping naked in front of him? Maybe for a bath?
9. How would he feel about rival gangs encroaching on his turf? Would he fight them or what?
10. How would he feel about being a crime boss in the first place?