Seriously, what's up with all the things threatening to destroy Azeroth outright?
The Old Gods seem to want to destroy Azeroth to get out from their prison within it. Algalon wanted to reoriginate the planet, which would have effectively destroyed the planet entirely and remade the whole thing, entirely to prevent the Old Gods from getting out. Now, with Algalon sitting back to observe, we had to step up and stop Deathwing from destroying Azeroth as well.
Granted, Deathwing was all tied up in the Old God's agenda, but you get the sense that old Neltharion wanted Azeroth destroyed because, ultimately, he couldn't take his connection to the planet's soil and rock, its very earth, for one more instant. Killing Deathwing almost seems merciful, since at the end he was nothing but a destroyed ruined pile of mutated flesh erupting from his elementium armor.