Capt. Brown didn't kill the guy. It was almost certainly a ground gunner, probably Sgt. Cedric Popkin of the Australian 24th Machine Gun Company.
I find it strange how so many people now still argue over this issue. An examination of the Baron's body found that the bullet had been aimed from below. Also, Brown stated "There is no point in me commenting, as the evidence is already out there" - I believe this was his way of stating that it was obviously not him who took down Richthofen.
The Baron had previously sustained a serious head wound earlier, so many people believe it may have had an effect - on that day, a fatal one - on his judgments. A super-pilot like him would've known better than to chase a novice adversary over heavily defended enemy territory.
it was confirmed that it was ground fire from an Aussie that killed him. One of those one in a billion shots from an unknown rifleman. A single bullet to the chest, angled upwards through the belly and chest cavity, and jusrt under the skin of the back. Says the the round was fired at range and passed through most of the body, losing momentum. If it was a AA machine gun or Brown's fighter, there would have been multiple rounds and through and through wounds, not a single lucky shot from a rifle round that was mostly spent.
Re., Natebros' comment: I think your illustration does a great job of depicting the ambiguity of von Richthofen's death--he is stalking Lt. May while Lt. Brown gets in position to shoot at him from behind, while he is at the same time flying low enough to be in range of gunfire from the troops on the ground. Canadians like to believe that Roy Brown shot down the Red Baron, since Brown was Canadian. Australians like to believe that it was ground fire that got him, since the AA fire came from an Australian battery. The Germans sided with the Australians, as they didn't like to think that their top ace could have been bested by another fighter pilot. From what I've read, though, von Richthofen had been suffering from a head wound or head trauma, and this quite likely would have impaired his combat judgement considerably, leaving him vulnerable to either danger. At any rate, I love your illustration.
as for the head trauma you are right, a silver dollar patch of skin was missing due to being hit in the head, how the round did not kill him is a mystery. he still kept flying when he should have been grounded due to wounds.