Trinity did not kill Rita
Ok here is the full reasoning:
Trinity as we know him had one motive that would drive him to do what he did: revenge. Dexter exposed him, and toppled his house of cards, so he strikes back by killing Rita. In this scenario, Trinity must not only find Dexter's real house (which we didn't see him do), go there, and kill Rita during the window of time she's actually there (she was going out of town). He must do all this while Dexter is desperately trying to find him. He spent a lot of time passed out in the back of his own van, making this very difficult. Furthermore, why spare the child if revenge is your only motive? He'd have killed Harrison, instead of leaving him artistically dripping in his mother's blood, exactly like Dexter. Rita was not a single woman, she was a mother. This was not a Trinity-esque kill, it was a Trinity/Dexter mix. Trinity had no way to know as much about Dexter as we assume Rita's killer does. This being the case, either Trinity knew more about Dexter beforehand, or it wasn't Trinity. The former seems unlikely since Trinity made no real moves against Dexter until he felt threatened. Thus we must assume the latter, it was not Trinity. It was someone who knows about Dexter's past
