Disappointed with final battle

I enjoyed this movie for the most part (yes it had its flaws but I looked past it) but the final battle with the father in law was very disappointing.

When he gives her the option to kill Dana, why the hell would he stand right behind her (in stabbing range)? And then he starts firing at her from awkward angles behind the boulder when he could have easily just stepped around the rock and shot her (she had no weapon of her own and was basically just cowering). Then when he finally does decide to finish her off, he abandons the gun and the knife to strangle her. Okay that's fine, but why did he plant the knife in the ground next to her hand?

The whole movie suggested the cult leader was an evil genius of sorts. Then he acts like that? Really turned the movie from having a cool ending to something silly.