SOME SPOILERS

As to Silva and his motivation, yeah, I think it was mostly getting revenge. In that sense, if M died half-way through, the end would, presumably be Bond just going after Silva. But I don't think that would be nearly as interesting.

As for it being a huge and destructive operation to get revenge against M, all I can say is, some men just want to watch the world burn. After Silva was "betrayed" he clearly had a mental breakdown of some sort and is now hell bent on getting revenge on M in the biggest, most explosive ways possible.

And, to further explain what he would do after he killed M, my best guess would be that he would kill himself.

ACTUALLY THAT WAS A LOT OF SPOILERS