What does Judges 15:8 mean?

Samson has sworn to avenge the cruel deaths of his former wife and father-in-law. This was the Philistine region's revenge for when Samson burned the crops and harvests around their town. That, in turn, was Samson's retaliation for his father-in-law giving his former bride to another man. Perhaps Samson felt some sense of connection to them. Or, more likely, he simply understood their deaths to be an attack against him which required a response. This typifies the natural cycle of revenge which Scripture warns we should avoid (Romans 12:19–21; Deuteronomy 32:35; Hebrews 10:30).

Regardless of the exact motives, Samson has naively decided this will be his last act of vengeance against the Philistines. The form this revenge takes is unclear. The text uses a Hebrew figure of speech implying total dominance, brutality, and fierce intent. Modern English expressions carrying a similar meaning might be "he went for the throat," or "he kicked their teeth in," or "he made mincemeat out of them." The literal words are not as important as the effect they imply. Whatever Samson did to the Philistines, it was vicious and decisive. Given that he'd already torn a lion apart with his bare hands (Judges 14:6) and killed thirty Philistine men (Judges 14:19), this probably involved a considerable number of deaths.

Samson seems to know the Philistines will come after him this time. Following his "great blow" against them, he takes refuge at a specific place. This is labelled as "the cleft of the rock of Etam." Most likely, this was a local landmark along a valley not far from Samson's hometown of Zorah. As expected, the Philistines will mobilize to capture him, leading to even more escalation and further bloodshed (Judges 15:15).