I started to lose interest during the repeated beatings, which is why the third act was where I decided it was a disappointment. It was too much like watching clockwork (dude comes down to punch her in the face for a while, chick comes down to feed her slop, repeat ad nauseum) for it to be shocking or scary. It's no wonder she achieved blank-faced "transcendence" -- that shit was mind-numbing.
The only appeal Martyrs held for me was the feeling I got from it in the beginning, where it paints this picture of a secret society of wealthy sadists, Hostel-style, and turns that on its head by introducing it as a revenge flick for one of the past victims. And here she comes, blowing everybody to fuck with a shotgun -- even the kids, oblivious to their parents' crimes. This shit is serious, right? It took a turn for the worse when the "creature" started showing up and haunting her in a cheesy, hamfisted attempt at confusing the audience into thinking maybe she's just crazy, maybe there actually is no secret torture chamber in their basement -- all this did was undermine the film's setup. So then she dies, and her friend finds out that the secret chamber really is there, undermining it yet again... and of course, she's stupid enough to go down and take a look.
And then yeah. Sorry, like I said, I wanted to like it.