Purifiers are an anti-mutant organization and religious madmen who take mutant-phobia to the extreme, this terrorist organization even went as far as destroying a bus full of former Xavier students despite them being de-powered. Showing zero remorse Purifiers see mutants and their allies as blasphemy against God himself and as such oppress and murder them without reason or mercy - they also see Scarlet Witch's depowering of thousands of mutants as a sign that they must push forward with renewed violence and exterminate the mutants that have survived.
The Purifiers are also major antagonists in the videogame X-Men Destiny and are major antagonists in Marvel Heroes.
God Loves, Man Kills (graphic novel)
In Marvel Heroes the Purifiers are a massive force that are fought in Mutant Town, where they mercilessly kill hundreds of mutants and seige the area - using abandoned school yards to stage hate rallies, burning down houses and generally engaging in blatant genocide on the streets.
They are opposed by SHIELD, NYPD and mainstream heroes yet also have a refuge in Fort Stryker, a military base converted into a hideout where they hunt mutants for sport with the aid of Sentinels.