Lumen killing Quinn is the easy way out, unless the writers find a creative way to deal with it.
My guess is that Quinn will find out who Dexter really is at the same time Debra does, and Debra will kill Quinn. Does it make sense? I don't know.
I do know--or at least am pretty sure--that Quinn is toast this year. The awful haircut and worse than ever spray-on-tan aren't helping his chances, either, since the supporting cast is mostly for standing around and looking pretty on this show (unless you're Masuka, of course). Combine that with an investigation of Dexter that's sure to come to only one conclusion, unless Quinn is shown to be unbelievably dumb (admittedly, his actions in the latest episode were pretty dumb, and a dramatic cheat to not get an answer from Jonah this early in the season/series).