‘ And he went to his father's house at Ophrah, and slew his brothers the sons of Jerubbaal, being seventy persons, on one stone. But Jotham, the youngest son of Jerubbaal, was left. For he hid himself '
Abimelech and his band presumably came on the brothers by surprise and took them captive, then they took them to a large stone and used it as an execution block. It may have been an official execution site. This may have been done officially on some pretext of treason, while the men of the town were in the fields, arriving back too late to protest.
Or it may even have been done as a human sacrifice to Melech, supposedly on behalf of his dead father (compare for such a stone for killing, 1 Samuel 14:33). After all his name was Abi-melech (Melech is my father). But normally such sacrifices would ‘pass through the fire', and there is no mention of fire here.
Similar activities to ensure accession by sons of dead kings were elsewhere an expected part of life, which would partly explain why there was so little furore. It was seen as an internal royal matter, and who was to argue with a son of the king with a strong band of mercenaries? All this is the more easily explicable if Gideon had officially been seen as their prince.
“But Jotham, the youngest son of Jerubbaal, was left. For he hid himself.” One, however, of the sons survived, seeing what was happening and managing to hide.