So, guy who never played 40k here but knows the memes. The red guy is a member of the Blood Ravens chapter, who have this weird habit of finding relics and wargear that are important to other chapters.

The common joke is that they steal them, which is illustrated here with him sneaking through the vents of the god-emperor’s palace with a bag of loot. There he runs into (I’m assuming) the necron Trayzn(?) The infinite, whose thing is stealing important artifacts and people for his little pocket dimension collection.

The Blood Raven naturally isn’t a total traitor or anything so he turns the necron in and just hopes really hard no one asks what he was doing in the vents to begin with.