Your facts are inaccurate. The crime was shooting at, not just hitting, the perp, so the shell casings were evidence (you think that you could shoot at someone and so long as you don't hit them it's OK).
The two supervisors arrived after the shooting, and did not know what had happened. One OTHER BP agent at the scene has resigned, and two others have been fired, for lying about the incident (trying to cover it up), in addition to the convictions of these two agents.
There's a lot of true facts out there, for those who want to go reading for them. The Inspector's report from DHS is now available, it's quite long and has most of the details in it. I haven't made it through much of it yet. Don't have a link handy.
Doesn't surprise me. My gut reaction is normally to sign with the police, however, they aren't above reproach and aren't above the law.