So if/when you did/do the quest at the end of the Klaxxi chain, you walk and talk with a Kil'ruk. He basically says the Old God's were awful, yet he says they worship them. Why then, do they worship them? Text below.
Kil'ruk the Wind-Reaver says: Wakener, your deeds have earned you the trust of the Klaxxi.
Kil'ruk the Wind-Reaver says: You are to be rewarded. Walk with me.
Kil'ruk the Wind-Reaver says: We mantid are an elder race. The pandaren you associate with - they are but children. They have their role to play.
Kil'ruk the Wind-Reaver says: Each cycle, our young swarm their walls. The pandaren slay the weak. The strong return.
Kil'ruk the Wind-Reaver says: With each generation, we grow ever stronger.
Kil'ruk the Wind-Reaver says: Before your history began, our empire was vast. We shared this world with our sister kingdoms, Ahn'Qiraj and Azjol-Nerub.
Kil'ruk the Wind-Reaver says: Our Gods were many, and powerful. We mantid worshipped the seven heads of Y'shaarj.
Kil'ruk the Wind-Reaver says: Great was the Old One, and terrible was His wrath. He consumed hope and begat despair; He inhaled courage and breathed fear.
Kil'ruk the Wind-Reaver says: When the usurpers came - the ones you call "Titans" - Y'shaarj was destroyed.
Kil'ruk the Wind-Reaver says: His last terrible breath has haunted this land ever since, but the shadows he left behind are mere whispers of his former glory.
Kil'ruk the Wind-Reaver says: I tell you now, because you have earned this warning.
Kil'ruk the Wind-Reaver says: Your gods are not your gods, outsider.
Kil'ruk the Wind-Reaver says: If the Old Ones ever return, we mantid will once again stand by their side.
Kil'ruk the Wind-Reaver says: The wisest among you will do the same.