I guess for some people it may be hard to believe, but people often suffer longterm shock rage and desperation after such traumas. Some never heal
That doesn't really matter. However enraged or shocked or traumatized one may be, unless that person simply lacks the faculties for rational thought, there's simply no excuse for killing random people for days on end based solely on an assumption.
Let's look at this in the reverse. If elven bandits killed a traveler just passing through, and that traveler's loved one learned of the elves' murderous crime, would he be right to go into the woods and simply kill every elf he sees? Hell, in my rage, that very same thought would cross my mind, but since I'm not a crazy person, it would occur to me that doing so would just mean I'm murdering people with no hope of really bringing the guilty party to justice, and I would be just as guilty as the ones that wronged me.