Though Gideon had rejected kingship officially ( Judges 8:23), though not practically, Abimelech desired it for himself. He also hated his half-brothers, presumably because he was the son of a concubine rather than the son of one of Gideon"s wives ( Judges 8:31). Shechem was one of the older city-states in Canaan. Canaanites were its primary inhabitants, as is evident from this story. They seem to have been even more open to having a king over them than the Israelites were ( Judges 9:6). As a local boy, as well as the son of Gideon, the famous military leader, the Shechemites favored accepting Abimelech as their king. [source][source][source]
"At least Gideon had said the right thing about God"s sole sovereignty: "the LORD will rule over you" ( Judges 8:23). Abimelech, on the other hand, leaves the LORD out of the picture entirely." [1][source]
Evidently Abimelech felt that Gideon"s other sons were ambitious to be king too, though there is no indication in the text that any of them felt this way. He was perhaps projecting his feelings on them, as is often true of ambitious people. They sometimes become paranoid, as Abimelech did (cf. King Saul). [source][source][source]
Abimelech was able to secure some popular and financial support by politicking. He hired some assistants who promoted him and probably helped him assemble and assassinate69 of his70 brothers ( Judges 9:5). He executed this slaughter on "one stone" ( Judges 9:5) suggesting a well-planned mass murder. Compare and contrast the similar story of Jehu"s slaughter of Ahab"s sons in2Kings10. Note how departure from God, idolatry, and self-assertion result in hatred and violence. [2][source]
The men of Shechem must have known about Abimelech"s slaughter of his brothers before they made him king ( Judges 9:6). Perhaps Abimelech"s violent behavior enhanced his value in their eyes. "Beth-millo" was the citadel in Shechem, the most heavily fortified part of the town. The writer also called it the tower of Shechem ( Judges 9:46; Judges 9:49). It may have been the fortress-temple of Baal-berith (cf. Judges 9:51; Judges 8:33). [3][source]
"The inhabitants of Shechem, the worshippers of Baal-berith, carried out the election of Abimelech as king in the very same place in which Joshua had held the last national assembly, and had renewed the covenant of Israel with Jehovah the true covenant God (Josh. xxiv1 , 25 , 26). It was there in all probability that the temple of Baal-berith was to be found, namely, according to Judges 9:46, near the tower of Shechem or the citadel of Millo." [4][source]
Abimelech was the first person ever to be crowned king in Israel, as far as the text records. [source][source][source]