The suicidal one-man armies of the Legiones Astartes, the marines that comprised the ranks of the Moritat were known to be a special case of psychological disorders and who abandoned any self-preservation, just to get to the enemy and murder them in the most violence-fuelled ways imaginable. Always attacking with determination that no other warriors in Imperial history had, Moritat Marines were the epitome of one-man army.
Originally being exclusive to the Raven Guard and being called there the Ash Blind (Sable Blind by the people of Deliverance), these warriors were veterans, who by some strange means, succumbed to a mental condition that caused them to fight with suicidal fervor and determination, as well as with a urge to murder the closest enemy. Most of these warriors died, yet those who survived were recovered and nursed back for later use. Corax organized these Raven Guard marines into Shadow Killers. Highly effective shock troopers and assassins due to their great usefulness. In fact, the Ash Blind proved to be so good at their job of anal annihilation that during simulated battles, the Ultramarines suffered a large number of defeats when they sparred with the sons of Corax when the latter deployed those suicidal maniacs.
Robbie, being the tactical analyst, looked at the Ash Blinds and pulled the same stunt like in the case of Perturabo and his Tyrant Siege Terminators when the former created his Fulmentarus Terminators. He declared the Ash Blind worthy of refining and thus created the Moritat. Unlike the Fulmentarus, the Moritat did find its way into every Legion. The Ash Blind mutation makes more sense than what two other Legions with genetic defects did to Astartes (and way cooler than turning into Furries or being used as cannon fodder). As it could be seen as a gene flaw that only comes out when a Raven Guard has a flash of PTSD. Instead of flipping out to Rip and Tear he loses his shit and decides to shoot or assassinate anything that isn't an ally.
Armed with an arsenal of Volkite Serpentas or Plasma Pistols linked to their jump-pack's power-source via feed cables, the Moritats proved to be very nasty warriors who could jump in, unleash a never-ending hailstorm of Volkite/Plasma fire and jump out to look for more prey. Volkite and Bolters are options when waiting for their handguns to cool down from the intense volleys.
So spake the fluff. The crunch is rather less forgiving to them, having been heavily nerfed since their inception and making them a rather expensive, limited, one-trick pony, only able to be joined by Destroyers who are also somewhat of a one-trick pony. Which is a pity because the models are baller.