Only a Sith deals in absolutes, dear.

The way I see things right now, revenge is a motivator which is at its core morally neutral, and what matters most in understanding it is simply cause and effect. One of my personal favorite historical figures, Ned Kelly, was very much motivated by revenge. He managed to garner widespread public sympathy on the one hand, but he also had his excesses and mistakes on the other, which ultimately resulted in Ned Kelly becoming a flashy folk hero without strictly succeeding in his political aims.