I think he is hiding something about what really happened to Paul, but i cant think why he would not tell the truth and tell us about what really happened to him

Because like a dumbass he instigated the worst of the beatings by shouting out at the thugs.

Thus he feels responsible for Paul's death and he doesn't tell the police about it because he feels he may have Paul's death put partly to him in their eyes.

Mitchells are natural born liars

No I don't think that at all. He was clearly being honest when he told Johnny bout the attack.