That's not even close to self-defense.
He's a Superman-grade brute. He's so invulnerable that the bullets don't appear to even be able to muss up his hair, they just bounced right off and didn't even make it to his skin. He doesn't get to attack someone with deadly force when he's not actually being threatened.
I'm sure you can tweak things (a la Nabu) to make him more sympathetic and the heroes more moronic, but in the comics, he never claimed self-defense; rather, it was pretty clear that he attacked (and killed four people) because he's a Lawful Stupid barbarian with a code of honor that demands that he respond to any attack, despite his associate informing him that they were lawman and the two of them should just back off and leave.
Also, those he killed? Not the one who provoked the attack. (In the comics, at least.) Greggson didn't die; it was three soldiers just doing their jobs and one civilian who paid the price for Konvikt to protect his "honor".
... and, sure, Paul can try to weasel out of things with the lack of absolute proof as to who was actually responsible for killing people, but Xor knows the truth.
I find myself perplexed by how Paul wants to hold heroes to the highest standards of behavior and decision-making, but villains just get excuses. It's like he grades the world on a morality curve, with heroes getting castigated for not being perfect enough and villains get praised for not being too evil.