Well I think it would suck if Batman would kill that is one thing he has ahead of the other Dc biggie Superman. I personnally don't think it would be any improvement if he kills it would just destroy an awesome Character, and correct me if I am wrong but did't he take a gun to go against the reaper in Millers year one (please tell me its Elseworld, oh pleaaaassseeee)
I think there have been a few Elseworlds were Batman killed, I think I remember the Nail for example. So sure your Detroit Batman sounds like a nice story (a little bit like thrillkiller maybe but anyway)
Oh and I think the other time when he came close to killing was during the Hush storyline when he thought the Joker killed hs friend (you know the doctor Thomas something)