Abimelech was not eligible to be king in Israel since he was the son of a Canaanite concubine.  However, his ambition to be king led him to murder his half-brothers in order to remove any rivalry.  Abimelech’s evil plan was funded by the Shechemites who used pagan temple resources.  After killing his brothers (with the exception of Jotham), the Shechemites then rewarded Abimelech as their king.