After speaking with spicy and getting the full situation on what happened here is my understanding of it,
Firstly you had many punishments, You and 3 warns for RDM then you RDMed again by seen within the video. After watching the video multiply times I have noticed the first time you shot you weren't looking at him yet you continued to turn and kill Copper, There is no need to blame hitboxs of this situation.
It is clear RDM Gmod and you need to learn how to deal with you consequences of your actions.