Gaal the son of Ebal had compared Abimelech with feckless Shechem, the son of Hamor the Hivite (compare Judges 9:28 with Genesis 34:2). To offer a better alternative to the men of the town of Shechem, Gaal the son of Ebal compared himself with Hamor the Hivite, who of course was the father of feckless Shechem. (Feckless Shechem ended up destroying his family and dynasty through his plan of intermarriage with the Jews.) In other words, Abimelech was half-Jew/half-Hivite ("problems will loom with him"), but Gaal the son of Ebal was 100% Hivite ("no problems will loom with me").
It is no better from the Jewish perspective. Abimelech was not entitled to any inheritance from his father --Gideon-- since he (Abimelech) was the son of a Hivite concubine. (Ishmael is a good example, since Ishmael's mother --a maidservant-- was an Egyptian concubine to Abraham, and therefore was not an heir.) What did not help was that Gideon named this illegitimate son "Abimelech," which means in Hebrew: "The King is my father" (or something akin to that). The TITLE "Abimelech" in the ancient world also carried the meaning of what we would call "head leader" or "crown prince." (Please click here.)
So Gaal the son of Ebed convinced the Shechemites that Abimelech was a racially mixed trouble-maker, who entitled himself (a mongrel Jew no less) to be king both of the Jews and of the Hivites of Shechem.