There really is no convincing some people.
Anyway, I'm sure that there is at least one attorney
somewhere in the states that would be willing to make that case. But that doesn't matter.
And it doesn't matter that the marines didn't have to intervene. That was perfectly legal. The stabbing wasn't.
It doesn't matter if there were 4 marines or 4 battalions of marines. If one got stabbed then the others can legally (by civil law) take measures to protect themselves and the injured.
The police report is probably the best thing you have going for you. Ironically it implies that the marines felt they went too far themselves, and of course one does not get all those injuries tripping on any curb but ones on highway overpasses. While it would make people suspicious enough to investigate, I'd have to say it's a bit beyond the scope of the true problem being discussed.
Let's ignore the rather stupid question of "Would they investigate?" for a moment and assume they do, and let's also make it interesting and assume the thief died: In most states, using lethal force in self-defense is permissible if you can reasonably show that lives were in danger and there was nothing else you could have done. In this case it depends on the fight. If the thief was disabled before the marines killed him then SD does not apply because the thief ceased to be a threat. On the other hand if he was killed on the first blow after the stabbing, then I'm willing to bet that SD would apply.
Now lets say the thief died the next day because of his wounds. Now the results of the case are determined solely by how much pounding occurred after the thief was disabled. The less there is the better the case the marines have.
Now lets just go assume the thief survived, albeit bruised badly. The same results as above. Things really only change if you take out the knife and stabbing.