There was a lot of time. When Dexter thought Rita was already gone, and him not on Trinity's tail at the exact moment.. Trinity must've gone back to the house, waited a bit or something and when Rita came back for her stuff.. he killed her.

It still makes me sad. T_T

Dexter had a few things to do, did he not?

Hmph...stoopid old man T.T

Also, the interview with MCH and some dude..

They talk about how if you knew she was going to die before even watching the episode, then the conversation between Trinity and Dexter meant something completely different from what you would've thought without knowing. Trinity was referring to the heaven Dexter wanted with Rita and the kids.. Saying it was already over.. It's so depressing.