Logical elmination of potential motives is pretty simple. All other logical pathways are eliminated by the empirical evidence of action, thus providing a clear deductive framework for the conclusion.
So, completely unfounded elimination of the motives that you can think of?
An active motive could be flying a ship for a differing navy. Enroaching on territory. Powerplay. BGS manipulation/protection. You could have had a target on your head (yes, there are successful, paid assassins out there). Pirating, and you weren't playing ball. Achievement of an initiative, such as barricading a CG. Testing new equipment/alpha damage tactics. And so on...
Or he might just enjoy shooting things. After all, Elite does condone "just hunting other commanders".
Frankly, grow up. It's a game. They are not suddenly a disgusting human for playing a game in a way you don't like despite having the right - which last I checked was what almost always gets waved around as justification for it actually being a problem? We can all play how we want, yes?
If you don't want others to play how they want around you...dude, you've heard this bit before. Open play is open so all the above can happen in a non-scripted environment. The only person coming out shamefully in such a bleak judgement of character...is you.