C'Thun

Old Gods are all yucky.

Well they're not exactly revered for their caring and loving hearts, actually I think the puzzzle-box of Yogg-Saron says the drowned gods heart is black ice.

It's interesting how dumbass mortals (players) kill these old gods without any thought of the consequences, since the titans themselves didn't kill them because they were too deeply linked to azeroth. And yet even with 2 of them now dead in the WoW expansions nothing happens to azeroth. Lorefuck.
One of the reasons WoW lore sucks. They should've continued the lore in RTS games.

The Old Gods were actually never slain only their physical forms were destroyed.

Then why was it so complicated for the titans to simply "destroy their physical forms" instead of going trough all the trouble to imprison them?

Because destroying their physical form can allow them to later reapear therefor imprisoning them was preferable.

Yet this shows that mortal races are dumb and are fucking around with things they shouldn't be and therefore they contribute to the destruction of their own world.

The problem is if an Old God was allowed to regain all its powers it would bring unimaginable havoc to the world of Azeroth. Yogg-Saron and C'Thun were about to break free from their jailors so annihalating them was the only option.