Who says they can't die? Besides, the Titans aren't your standard (im)mortals either, I'm sure they know a few tricks to take care of something as evil, ancient and (im)mortal such as an Old God. Maybe they didn't kill it in a way that can be described in terms of living or dead. They split up the Old God into lesser beings of evil, the Sha. Destroying the physical body and tearing the soul apart. Since that is nearly as bad as leaving them alive, they imprisoned the other Old Gods and put them to sleep.