I dunno. Maybe my real slave didn't follow me out, maybe it was on manual mode, or was blocked, or maybe Player Slave is loved by the city and can attack slaves willy-nilly, maybe it's a minor glitch, it's happened to other players before. You know what? It really doesn't matter how it happened. All of that is beside the point.
The point is, Slave is a misleading name. I suggest that, to clear it up now and forever, just make it so players can't have names of NPCs that join your party. It's that simple. "But Ryvaeus, what if I want to RP a slave?" Then call him "Slave Bob." "It takes away from the immersion!" So does having names like "headshot1234." Besides, we already can't have identical names with other players, it's not like it's such a big deal to also not have identical names with NPCs that are recruitable.
This guy had 200+ HP, it's quite clear that this confusion - I practically consider it a scam already - is what he takes advantage of regularly.