“to Ophrah.” For Abimelech to murder all of Gideon’s sons right in Ophrah near Gideon’s house is incredibly coldhearted.
“70 men.” This is the number of all the sons of Gideon, 70 men (Judg. 8:30). Two were not killed. Abimelech instigated the murder and Jotham escaped, so there were 68 men murdered.
“on one stone.” The fact that 68 men were executed on one stone shows they were not killed while defending themselves, but were captured and then sacrificed, almost certainly to Baal. Occasionally human sacrifice was made to Baal. A sacrifice of this nature reveals how ruthless and cold-hearted Abimelech was.