He was dressed to kill, and had the needle, but as far we know, he didn't have a kill room set up. He could have been planning on asking Quinn questions or scaring him or just making sure he destroyed the evidence. Having said that, there was at least one other innocent guy killed outside of this season, when he made the mistake on that guy who was running a similar snuff film operation (his partner was actually the murderer, can't remember what season). Anyways, I've never gotten the impression that Dexter is particulary bummed out when he makes a mistake, or kills someone in self defense, and rule #1 obviously comes before any other, so I think it's safe to assume that he'd kill almost anyone to not get caught. And you're right, helping Lumen acheive her goal seems like an added incentive. I'm betting Deb at the very least finds out about Dexter/Lumen's involvement in these murders. Unfortunately, since every single season special guest star has been killed in the last episode, I'm expecting Lumen to go down, either changing Dexter in the process, or enraging him further and next season he'll have quit his job, sent the kids to the grandparents, and just hides out in a park killing people under a bridge all fucking day. I'd totally watch.