IMO, the principled libertarian position is to not really have a position. I'm not a court a judge a jury or the family of someone who was killed, so why does my opinion really matter at all?

Things like this are similar to abortion to me, where I can have my personal beliefs of what seems right to me, or what I'm comfortable with, but not necessarily demand that other people do it the same way.

I don't really know how I'd feel if a loved one were killed by someone, but I don't think I'd feel any kind of satisfaction in killing back. I'm sure I'd want them excluded from civil society and for everyone to know what they did. But that seems like the most you can do. Sometimes a situation just isn't correctable.

That said (circling back to my first paragraph) I wouldn't try to stand in the way of people or courts who support the death penalty. It's probably not what I believe is the best way to respond to the situation, but I'm not going to try to make everyone else follow that belief.

If someone wants to kill back, perhaps that's the best way for them to grieve their loss, and perhaps it's best to let them.