The only way to understand the sudden change of character in Takumi in episode 11 and 12 (and to salvage what was otherwise a psychological thriller) is to interpret the events in episode 11 and 12 as nothing more than what took place only inside Takumi’s head—instinctive mental self-defense if you will.
Somewhere near end of episode 10 shortly after the final crack on his mentality broke open as his sister disintegrated in his arms, Takumi had completely lost it. Unable to get a grip on reality, he retreated irrevocably into his mind where memories of video games prevailed with protagonists powering up, defeating the big bosses, and always getting the girls at the end—things that he, useless and weak as Rimi had pointed out, could never achieve in the real world.
Haven’t we all done something similar ourselves? Back in my high school days, I constantly daydreamed how I would bravely (uncharacteristically of course) beat up and humiliate the bullies in school and get the heart of my dream girl watching me do so. In my daydreams, I was always brave heart and iron fist.
Only in a daydream can a person’s character be as drastically changed as what took place for Takumi in the last two episodes.
Of course, you’ll object, but the show didn’t conclude with any hint that the ending was all a delusion. Well, um… err… hey! Look at all those Seira-tan! *runs away in denial that the writers fucked up this show*