This is wildly improbable. If I shot myself on the left side with my right hand (who would even think to do this? It's really awkward), the pistol would drop to my left side, and my arm, most likely, would land across my torso, not on the right side.
I have pretty long arms (35" sleeve), but I don't think I could do this with a pillow. The gun was a Glock, which means the shooter's hand would have to have been about 5 inches back from the body, plus another 3 or so to account for the pillow. The angle of the bullet was such that the shot entered really from the left side, not at an angle; about perpendicular to the front of the body. I just can't envision this being a suicide.
The Only way for it " drop to my left side, and my arm, most likely, would land across my torso" is if there is absolutely no recoil, your body does not move at all and you immediately drop the gun. That will not happen - the gun will recoil, and it will go straight back at the angle it is held -- A shot instantly killing you (hitting the heart) Will cause you to immediately lose your grip, but at that point your hand will already be moving in the same direction as the recoil of the gun.
LIke I said, i have not seen the angle at which the shot entered Helgoth's body, if it was angled to the right (from the left) then the gun will land on the left side. if it is angled across the body from the right, that gun will recoil and leand on the right side of the body.
I do however as I said, think your 2 man scenario involving Helgoth is very plausible. Hopefully they, or someone, is looking into possible links between helgoth and Karr (maybe internet chatrooms/news groups etc at that time)