Revenge in Star Trek is a trait that may often be attributed to the villains but it is by no means limited to them. In every bright and shining universe, there is a dark side just waiting for its chance to come out.
This is as true in Star Trek as it is in any franchise. Gene Roddenberry's future maybe have been created as a Utopia, but even he admitted that Humanity would go through Hell to find its way to Paradise. The Bell Riots and World War Three both occur in the build up to the future as we've seen it, paving the way for allegedly better times ahead.
This list breaks down some of the most shocking and brutal acts of revenge to ever occur in Star Trek. Often times these acts come from cold calculation and long term planning.
Other times, they are hot blooded and come from seemingly nowhere. In any case, in a universe that prides itself on its inherent goodness, the existence of vengeance in the future is proof enough that even with the best of intentions, humanity and its cohorts cant shake the basest elements of their nature.