I'd say Dumbledore's responsibility for Riddle lies more in, as assistant Headmaster, sending him back to the orphanage each summer, feeding his anger and driving farther into the Dark rather than making -any- effort to try and pull him back toward the light (It might not have worked, but it should've been tried unless he had na ulterior motive, even then) and then, later, when Riddle applied for the DADA positon, not dealing with the whole of what he saw at that time, since Riddle was already well on his way into the deep Dark with at least one horcrux already made.
I suspect the final blow will be a primarily muggle weapon delivered/used by Harry. I would hope, though, that it's not the suicide bomber approach, after this is over, he deserves some good times.
Author's response
Well there is that, yeah...