You enter a room and see before you a still body on the floor, apparently dead, bullet-ridden it would appear. Close by, smoking gun in hand, there is another person looking at the body. The building is probably empty. You heard gunshots and ran in, up the stairs to the room whose door was slightly ajar. The person with the gun lets it hang loosely at his side. You are in no immediate danger, you sense. What has transpired? Let us name the person on the floor, B; and the person holding the gun, A. Did A shoot B?
Many would probably say this is the most likely, most probable explanation of what's been observed. But have we not made a few assumptions? First of all, is B really dead? Did B really die from gunshot wounds?
Things may not be what they appear to be. Perhaps B was stabbed to death and A was shooting at the person/s who did it trying to save B's life but was too late. Perhaps B is unconscious, gravely wounded but still alive - you should call an ambulance immediately. Did A kill B? Perhaps B tried to commit suicide and A arrived on the scene, and picked up the gun for whatever reason. You don't know exactly what happened. You can't just go and shoot A; especially since you have established your life is not in any immediate danger. The situation may change suddenly. A may have been the perpetrator of the crime. Call the police, take evasive action.
The point I'm trying to make is that jumping to conclusions can land one in hot water. Making assumptions can lead to very negative outcomes, especially if acted upon.