Unless they had a tool or RemoteEvent/Function they could use to deal damage to/kill a player, it should be impossible for this to happen (from my knowledge).
If you want to prevent it from happening, just use sanity checks where the client interfaces with the server. Make sure they’re not too far away to deal damage, they’re not dealing too much damage, etc.
It seems like a custom death system, so there’s probably something going on there (e.g. client telling server to kill humanoid since the health is at 0 for the client)
Yeah. Also, maybe you could retry the video but instead you code the local script in studio and not during runtime. I don’t think this will change anything but it’s worth a shot.