Unusually, he’s a Terran who had defected to Mars together with his father (who we never see) because his father wanted to study Aldnoah^A). He mirrors Rayet in this way, although I haven’t done much analysis here yet^B). Due to being saved by the Princess, he has a crush on her.
He had been continually abused by the Martians around him for being a Terran, so he entirely logically decided to become their leader.
He goes from staying with Count Cruhteo to staying with Count Saazbaum because the latter murdered the former. However, Saazbaum angered him after he shoots the princess so he pledges his allegiance to Saazbaum? After a brief time staying with him he murders Saazbaum after he had adopted Slaine, as a result Slaine inherits all of Saazbaum’s things and becomes the leader of the Martian invasion.
When you actually think about it, Slaine was pretty insane^C).
His story ends when Inaho captures him. So he gets imprisoned and blamed for everything. The media declares him dead. It’s depressing until you realize that he’s a bastard, and death probably would be what he deserves.