OK so i have a PVE carnage build and this may be off topic but rather than working out the damage differences I re added Vicious Slash to my rotation. After many attempts I figured out my output dmg wasn't high. My rotation was battering assault, gore, massacre, force scream then spam random moves till cool-downs came off. I then reworked vicious slash into it. since massacre leaves a buff I figured during that buff id spam vicious slash to get the extra damage. As a side note SPAMMING MASSACRE IS A COMPLETE WASTE OF DPS. so my new rotation is now batering assault, gore, massacre, force scream, then use vicious slash only until other cool-downs are up and massacre buff is on I then repeat. therefore i don't face-roll any more. With the massacre buff up I have the 1.5k dmg from vicious slash then i get the extra 900 from ataru form strikes. If you use just massacre you only get about 800 dmg plus the guaranteed 900 which is less. If you use vicious slash only when massacre buff is up you get a some 65ish% chance to get a 900 dmg hit giving this a lot more dmg output.
I hope you guys find this useful in some way, it definitively has made my dmg increase by a ****-ton and a lot easier to solo.