One possibility could be that Diesel's former lover was an Alpha of a wolf pack. Diesel was the Alpha's mate and therefore Diesel would be with him regardless of how he was treated. Most animals work on a heiarchy of sorts; If you kill the Alpha, you gain it's mate and you become the leader. This is why Diesel said he sometimes wonders if he's a slave to his instincts or if he's actually in love. His instincts as a wolf tell him to be with the new Alpha (aka, the killer). He also doesn't know if it's love. It's probable that Diesel's past lover was a terrible person but he still loved him anyway.
That makes sense, but still to have a relationship with someone that killed someone you knew and was involved with seems weird. Unless like you said Diesel's former lover was abusive. But I also want know the reason diesel's current lover (can't remember his name) killed the former one.
I'm sorry but how is Diesel with someone that killed his former lover.