I really didn't see that way because Slaine basically threatened to kill Inaho if he touched the princess and when Inaho pulled out a gun on him Slaine shot him at that moment. If Slaine wanted to kill Inaho he would have done it from the get-go during the time of the whole crash landing scene . Past episodes really didn't show him like he wanted to kill Inaho either. Or the writers probably didn't mean anything by it and shouldn't be taken at anything deeper.
Slaine had snapped. He just saw the Princess completely ignore him in favor of Inaho and then his own actions were responsible for getting her killed. At that point he had made up his mind as to which side he was on.
It was sorta like that desert scene in Breaking Bad. The guy's mind had clearly already been made up.