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.
Examples include:
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)?