No, the title is not an oxymoron. A great plot hook for a campaign involves a pacifistic movement with dark beliefs. Nothing will drive your PCs crazier than a group of people that will not fight back, even if you start cutting them down. All that is required is that the pacifists follow some belief system that is polar opposite to the prevailing view. For example, the movement treats women badly, worships the wrong god or otherwise offends the general population. It does not really matter so long as the issue is worthy of the adventurer’s attention and efforts.
It is important that the pacifists not practice “real” evil. If they regularly eat babies then cutting them down, pacifist or not, is probably a real option for most PCs. Make them a social disruption, not the harbingers of evil.
Fireball and sword do not count for much against people who do not fear death and refuse to fight back, so the PCs need to broaden their options. Perhaps the party infiltrates the movement, with sabotage on their minds. The monkey-wrench need not be a physical one. I like the idea of the PCs working from the inside to destabilize the movement. Exposing some sordid details of the leader, blackmail and outright propaganda are all on the table.
My personal favorite is a counter-movement run by the PCs! Create a schism within the movement and watch chaos ensue.
I will not say this type of campaign is easy to run. It would require a great DM and clever, motivated players. That said, it would also be a lot of fun.
Trask, The Last Tyromancer