One thing I'm grateful to my Dad for is when it came to my relationships, he wasn't one of those helicoptor Dads who chases off every male who comes near. He's the same with my little sister, and in fact gets along with my future-brother-in-law-to-be. He's always let me make my own decisions, while being there in case I get hurt (although he did tell my ex that I came with a no-return policy. Thanks Dad).
I don't understand these fathers who threaten their daughters dating prospects, even when they're younger. All it's going to do is breed resentment on the part of the daughter, and encourage her to go around behind her fathers back so she can at least experience what love and relationships are all about. Plus it's just plain sexist in a way - if you're going to kill a guy for touching your daughter, you should at least then be willing to hand over your daughter to the guys parents so they can kill her too for touching their son. Not to mention it gives your daughter an unhealthy example - she will learn that males be controlling is okay, and that will affect her relationships negatively.
Parents have a responsibility to their children to model behaviour to their partners that they would like to see in their children, and how they would like to see their children treated. If a child sees a father who is domineering and controlling, sons will become domineering and controlling themselves, and daughters glean that they are to submit, potentially landing them in abusive situations. It's the same the other way around, if the mother is jealous and controlling, it will flow on to their children.
We learn a lot about relationships from what is modeled to us, and what I have learnt from my parents isn't all good, but most of it is. What I've learnt from my father is that being open and honest without controlling is a good way to be, and from my Mum I've learnt that it's okay to walk away if things aren't right. I might not have always put it into practise, but after my last experience I think I know where I went wrong and what not to do next time.
So to all those Dads out there feel they have to threaten every male who looks at their daughter, relax. You're going to have to let her go some time, and if you've done the best you can, she's bound to find someone who will treat her like the princess you think she is. Just remember to be there if she falls, because there's just some things only Daddy can fix.