[Malek]

Originally Posted by mr_j

the police chief reveals a wallet, no id or anything unusual was in it, except for a picture of Crystal soul.

Although Billy was angry because he wasn't promoted, and Crystal Soul is his wife so it's normal to carry her pic in his wallet , i don't think that he's the assassin.
I think that it's Sogelec, since he still loves Crystal Soul and Billy is his best friend so he killed Alias because he didn't deserve to get promoted(unlike Billy).