In the war between the Titan and Old-gods, the Titans couldn't kill the Old gods they just neutralized them and sealed them. And even in their weaker form they are still powerful.
C'thun created the quiraj and turn Neltharion mad.
N'Zoth created the emerald nightmare .
Y'Shaarj is supposedly slay but his heart is still beating .
Yogg-Saron created the faceless ones.
In the game you can make a raid to kill C'thun and Yogg-saron, but are really they dead in the canon plot (like Arthas or Deathwing)?