you don't need to identify the slasher. the head orc does that for you.
there is no way to stop the orcs from killing you bodyguard. what you can mabey try is run in so that the zombie is far behind you and then talk to the head orc right away and give him the slasher. if it is the slasher then all of the orcs will head up and be gone. then you can keep your zombie. i am not sure if this is going to work but you can try it. save before you try it. and you don't really need the zombie. he will get killed 1 of 2 ways.
1. you go above ground.
2. the bigger skeletons you meet will kill him.