OVERKILL MAXXING
 Hive Scum Build - Warhammer 40k: Darktide 

The recent Decapitator buffs changed it completely for the better, it is now the best damage boost by far on a Brutal Momentum Combat Axe.

During Rampage!, Rashad Mk III Combat Axe Light attacks will one shot headshot and cleave through any human sized Elites except Scab Mauler, which will only be one shot if you built up enough Hyper-Violence. 
And yes this build has insane amounts of Overkill damage, but thanks to Hyper-Violence it will not be wasted, as long as you find some bigger targets to stick the axe into every so often while clearing a horde.
Without Rampage you will still one shot any non-Elites anyway, so keep it for Elite packs where it lasts longer too.

Heavy attacks should one shot Bulwark and Reaper even without Rampage up, and you don't need any rending on this axe to deal full damage to Monsters and Crusher either, just use the Heavy attacks.

Also, during Rampage you will have enough attack speed to build up Hyper-Violence with Heavy attacks instead if you need to, such as against lots of Scab Rager and Scab Mauler.

In my testing you can't really get oneshot breakpoints for all Elites without Rampage active, even if you use Stimm with Wildfire V, so better to use your Stimm just to get the Rampage back quicker instead, with Kalma V or Hypex .

I think you don't need the Needle Pistol for Crushers and Bosses, so the Branx MkVIII Dual Stub Pistols can be used to easily clear specials instead.

The talent tree only has the points allocated which I consider a MUST, so you have 4 points left to allocate as you see fit.
These are the 4 I use: In Your Face Sticky Hands Slippery Customer Jittery 

Usually I would use Nimble instead, however I feel like the Combat Axe could really use the longer and faster dodge from Alley Rat, just so you can get around faster while swinging.