Occam's razor says you should not multiple entities beyond what is necessary.

Our scenario is this: There's a guy in a vehicle. He is holding a gun. He has a gunshot wound. He has been under a lot of stress.

Explanation 1: The guy shot himself. Number of entities (people) required: 1

Explanation 2: Someone else shot the guy and staged the scene to look like a suicide. Number of entities (people) required: 2

Occam's razor says suicide is the most likely explanation