After the Emperor announced that he would be leaving the Great Crusade at Ullanor, there was a never before seen level of butthurt in the galaxy. This butthurt lead to the formation of a daemon entity in the warp known simply as the "Butthurt God." In much the same way as Khorne is empowered by bloodshed, regardless whose blood, the Butthurt God is fueled by any and all butthurt. At some unknown point between Ullanor and M41, enough astartes had been corrupted by the Butthurt God that the warband of the Butthurt Marines formed. Their ultimate goal is to invoke butthurt in all those they encounter, including their own battle-brothers. The targets, however, extend beyond the galaxy and into the fan base. The corrupted astartes will attempt to make 40k fans butthurt as well, in order to make their master more powerful. Their main, but not only, means of achieving this goal is through their love of Games Workshop writer Matt Ward. They admire Matt Wards gift to invoke unrelenting butthurt in 40k fans and they support his efforts in order to create even more butthurt in the fan base.
Without further adieu, the Butthurt Marines' insignia and a drawing especially for you all:

