According to the author of Judges, this verse tells a story about the Benjamin people fighting in the same way they did before, and they made some of the Israel people get hurt and fall near the roads. It's a story showing how the Benjamin people were brave but also made choices that led to more fighting.
Judges 20:31: And the children of Benjamin went out against the people, [and] were drawn away from the city; and they began to smite of the people, [and] kill, as at other times, in the highways, of which one goeth up to the house of God, and the other to Gibeah in the field, about thirty men of Israel. [to smite...: Heb. to smite of the people wounded as at, etc] [the house...: or, Bethel]
Judges 20:31 describes retribution and warfare, whereas Matthew 5:44 advises to love enemies and pray for those who persecute you.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;