What would you do if your horse killed your relative?

i would hunt it for sport 

It would be put down. I have seen horses that will try to kill anything that comes into their paddock other than a human like cats, dogs and chickens. I saw a horse try to or was going to kill it's owner by pressing the person to a wall. The horse had it ears pinned back and wringing its tail and leaned into the person and pinned her to the wall. If it had not been for someone running over and walloping that animal the owner would have been hurt bad or dead. Ill tempered animal, it went bye bye.

Depends if it was nasty or how important my relatives is if I like them or not

Depends on if it was an accident, human error, or if the horse was being aggressive. (And honestly it would depend on the relative). 

If it was an accident, I would be upset but it’s not the horses fault. 

If it was rider error, then I would be upset but not the horses fault or my fault. 

If it was my error, then I would feel horrible and guilty and beat myself up, but not the horses fault.

If it was the horse being aggressive, I would look closely at what happened and try to understand if it was something that could be fixed. And most insurance companies would make you get rid of the animal since they are considered a liability. I wouldn’t give a dangerous horse away, so ultimately I would probably decide to have it euthanized. 

If it was someone in my family I was close to, then I might not be able to be around that horse again just out of my own issues, I probably wouldn’t sale them either, but if I wasn’t close with them and the horse wasn’t at fault, then I’ll probably just be upset and redo the basics and start the horse over to ensure that everything is really okay and rebuild my trust in them. 

Depends on the circumstances.  Context is everything and you gave none.

P.S.  I know of someone who was way too fat for a horse who got on anyways and the horse went down and rolled over on her killing her.   Should the horse be killed because some 275 pound idiot plus a 50 pound saddle got on a 850 pound horse?

Depends if it was an accident or if the horse was intentionally trying to hurt somebody.

Human>animal that horse would get put down for being violent