nah, Dexter had every reason to NOT kill the trinity killer, he was trying to learn how to be a family man and carry his dark passenger... He thout TK had it all figured out, didn't realize how broken TKs family was until the latter part of the season
He had already done that for 3 seasons and certainly didn't need a serial killer to help him do so .
It made no sense, just like Dexter killing somebody who wasn't a killer and then forgetting about it (along with the show) despite the show being built on him only killing other murderers