If Lundy's death is related to the Trinity Killer, it could be that Trinity has multiple personalities. Episode 5, the "Denver Omelette" scene features Trinity looking shocked at the news of Lundy's death, immediately followed by anger (he also mumbles something while watching, not sure if he's reading the screen or what). Debra had been meeting with Lundy to discuss the TK case quite often.
Does anybody really believe that the Trinity Killer is falling for the "Kyle Butler" routine? If Dexter truly believes that Trinity shot Deb & killed Lundy, doesn't he know that more than likely Trinity knows who Dexter is already? I mean, he was hand-picked by Lundy to help on the TK case and he is Debra's brother (who was also shot). Wouldn't it be easy to figure out who Dexter was since he's so closely related to Lundy & Debra?
It's very possible that Trinity knows alot about Dexter. Wounding Debra could be part of his plans for Dexter. It's also possible that Trinity's son could be involved, that would ultimately make Dexter concerned for his own son's future. Perhaps Trinity's son shot Lundy to save his dad?
Knowing how this show normally works, it wouldn't make much sense for the killer to be the news reporter, Anton or Quinn. I'd pick Matthews over those three (he's evil, it's just a matter of time). One thing we know is that Dexter's character has to evolve after Trinity's death, so we can expect some enlightening twist.