Why did Kronos kill his father

Revolt of the titan son Zeus against his father Kronos

   

In order to prevent him from being overthrown by one of his sons, as predicted, the Titan Kronos devours all the children born to him by his wife Rhea. Only Zeus can hide Rhea from her husband by handing him a stone tied in bloody diapers in place of the son. The grown-up Zeus then actually challenged his father and won. The fight of the titan son Zeus against his father Kronos is called Titanomachy (titan fight). Zeus was supported by the Cyclops, from whom he also received thunder and lightning, the Hekatoncheirs, Styx and his mother Rhea. Kronos was supported by some other titans, namely Iapetos and Koios, who were banished to Tartaros by Zeus after the fall of Kronos. Kronos himself banished Zeus to the island of the blessed, where he rules together with his wife Rhea. Since Oceanus, Tethys and Themis did not take part in the battle against Zeus, Zeus left them rulership over their ancestral realms.

(Source: http://de.wikipedia.org/wiki/Liste_griechischer_Sagen)