Yes.
When something is taken from somebody who committed a legal act (self defense) and not returned IS theft.
If God forbid somebody has to use their gun in self defense, it should be returned to them the moment it's established at the scene that they're the "good guy".
People in stressful situations do odd things. That is not a criticism, but an observation.
Samaritan: Trooper, are you ok?
Trooper (getting his head bashed against the pavement): Gghhruhdhun...
Douchebag: He's fine, we're only messing around. We're really lovers.
Samaritan: He doesn't look fine.
Trooper: GHUHHRHUSHUGNG
Douchebag: This isn't what it looks like. He was banging my wife.
Samaritan: I think you should let him up now, he's doesn't look right.
Trooper: GHhugh Kuhruhr hifhfurh
Douchebag: See, he said he's fine.
Samaritan: Look, I'm getting my gun.
Trooper: SHOOOOT HIIIIIMMM