Yeah, I'd probably ditch that entire last part, I'm also concerned about it being any Character, Monster or Vehicle, as 8th has a lot of weaker "add-on" characters like standard bearers, medics and such, which could be very easy for a Khorne Daemon to kill, plus some Monsters and Vehicles aren't actually all that tough, depending on the Khorne character and their equipment.
Personally I would limit the bonus to only the daemon itself, who can only attack the selected target if they want to keep the bonus (permanent once target is slain). As for targets, I'd maybe limit that to HQ
choices, or maybe factor in power rating to exclude chaff Characters. You could maybe let a Lord of Skulls do this against other super heavies, but I dunno how best to word that concisely.