This has been driving me crazy for a week now. There was this movie, my guess is that it came out in the past 5 years or so, about a girl who kills a bunch of people more or less in a revenge kind of way. I don't recall much about it but:
- the lead actress was young (20s) and not well-known but there was a better-known actress as a secondary character (can't remember who or I would've found it)
- lead actress kills someone with a sniper rifle, through a window, while said someone was threatening or hurting the better-known actress mentioned above
- the ending of the movie is the lead driving down a highway and getting stopped by a sherrif or state trooper, who comes to her window and she proceeds to stab him in the neck (and then shoot him I think), as it turns out it was someone from her past who hurt her as a child or something

Can't remember much else, sadly. It's a thriller and she doesn't kill TOO many people, but the scenes are pretty graphic when she does. And it wasn't something super-targeted, she just starts killing people who hurt her or her family/friends. There was also a scene when she learns to fire the sniper rifle I think.

Thanks for putting up with me, hope this helps.

P.S.: It's definitely not Frailty.