Here's why:
* Ares's motive: Ares wanted to start a war between the gods and the titans. He believed that a war would give him power and glory. By getting Luke to steal the Master Bolt, he hoped to spark a conflict.
* Luke's motive: Luke was angry at the gods and felt betrayed by them. He believed the gods had abandoned him and was looking for a way to fight back. Ares, recognizing Luke's anger and need for power, manipulated him into stealing the Master Bolt.
While Luke did become a key player in Kronos's army, he was not directly working for Ares. Ares was using Luke to further his own agenda.
Hope this helps!