I'm not necessarily interpreting the incongruity/implication stuff I mentioned as flaws. My post might have been vague. What I mean is that, while the final act is essentially a tasteless joke, I consider it to be a very interesting one for the reasons I mentioned up above and, while I think the tastelessness still exists in the final act, the thought put into it lingered with me for a while and that's what I find memorable about the final act. I consider the final act to be an interesting tasteless joke.
While I may be misinterpreting what Crummy was saying, in which case he can feel free to correct me, I think that he was referring to the molesting with a pistol scene as a tasteless joke.
I think the ending is a very clever subversion of horror tropes (something we agreed on) that inverts the final act of the slasher and forces us to view several different concepts (notably family and justice) through an unexpected and provocative lens.
I see that as a well intentioned feature of the movie rather than an accident or mistake.