He was shot and there aren't any double blind studies that prove that police are capable of reliably distinguishing self inflicted wounds from non-self inflicted wounds.
I thought firing a gun would leave gunpowder residue on your hands
I'm pretty sure there are other ways of putting gun powder resedue on someone.
Yes, the point is that unless they actually take the time to do something like that, you might see that the victim has no residue on their hand and could not have shot themselves.
If you are an organ donor, it will surprise you what kind of research is done using corpses.