Hive Scum "Support" Knife Maniac - Share you're drugs
 Hive Scum Build - Warhammer 40k: Darktide 

I have a few variations of this I go between, but in general this feels really good for me and I reguarly outdamage both psykers and vets in H40, while also supporting the team. 

Finesse on the stimm tree gives you the most benefit for stabbing things with you're knife, but going rending for the team is also good. 

I dont know if this is perfect or ideal, and im sure an argument can be made to swap a few points around, but in general this is what I've landed on tuning this over a dozen or so h40 matches with it. I've found this is a sweet spot between crit chance and dmg nodes and utility nodes - you play 50/50 with the knife and pistol. You can dive in and stab things like a maniac and its great against elites, bosses and armor. However you're weak into big hordes - this is where needle pistol alternate fire and maybe a rocket comes in when things get super spicy. I like to shoot rockets at big armor blobs, bulwark blobs, or giant reaper spawns that are buffed and dangerous. Also boss situations but only if there is also elites and specialists around the boss for more value, else you can just poison stack and push attack the boss to death. 

You play with your speed, rushing gunner lines, taking care of specialists quick, going for quick revives and stimming the group. You can play vanguard, but also play more upfront being careful around the ogryn/arby/zealot not to eat any of their overheads, and support them with damage and eliminating key targets. 

This build is no slouch and isn't merely a 'support' build it regularly tops charts when played right because Hive Scum is OP lol