Morty: I figured it out, Rick. The parasites can only create pleasant memories.
The first episode of season two tells us just how sure of himself Rick is, and I don't doubt that what he says is correct about him buying 3DS systems:
Rick: Okay, yes. I definitely remember doing that. But also, I would never do that.
So sure of himself, he must be, that even this doesn't convince him to start shooting family members. After all, there's a hundred percent chance of the possibility that in this universe, he did do that.
Ok, yea: he tried to wing Summer - but we're not here to point out plot inconsistencies. Besides he's, "not used to being this unsure for this long." It's probably at this point he realizes that's the only thing he's sure of, which belays any further action.
Regarding the ending, we have to look at
the modified intro sequence that shows Mr. Poopybutthole in some decidedly unpleasant memories, the only pleasant one includes Mr. Beauregard.