Making and Playing a character That's both Evil and Good by nature at the same time

Hello people.

Look, I have this Dhampir Figther guy, that is the oldest idea for a character that I've ever had, and the thing is that, even if I know how and who he is, who I want him to become, and all, the idea since that it's a hell to roleplay well.

So, our guy, at the begining is your old timey, idealist knigth, with his chivarly and his extremely good manners, helps the poor, has epic good dialogue, and it's basically a chivarly's novels best dream. but that's until cetain moment in the story, whenever the rest of the group agrees, they are going to notice the group doing suspicius stuff, knowing things he shouldn't know, know everyones dreams, and walks on walls, and after some stuff happens he gets slapped with who he really is (BECAUSE EVEN HIM DIDN'T KNEW, this is important, is not a twist villan, nor a traitor, or anythig, it's still a party member, I don't wanna be a murder hobo).

Long backstory short, he was a rigtheous adventures that got bitten by a Dhampir a lot of centuries ago, got brainwashed, turned into your classical evil manic egotistical powerhungry vampire, that killed hundered of adventurer and ate their dreams, this is important, because after a lot of centuries of doing the same thing, lonley and bored those dreams craked his very mind and turned him, not into his pre-Dhampirification self, but into the flanderized version that him, as an evil pridefull guy, learnt to see adventurers as.

But when he remembers, I want both of his real selfs to exist, the rigtheous adventurer blooms again after centuries, the man that does good thing, not only because it's the rigth thing to do, but because it would be against his very being to not do so, and the absolute bastard that liked being a bastard.

You see the issue?, past this revelation we are having a guy that enjoys both the suffering of everyone he deems lesser than him, and the smile and protecting of those that are weaker and cannot protect themselves, the man that hates innocence and would do anything to protect it, both at the same time. HOW?!