TL;DR: decap should get a stack if damage >= target’s max hp
I’ve been using combat axe on arbities for a couple weeks now and, all things considered, it’s kinda meh. Attack speed is slow, cleave is bad, but the damage is great and it can make some very nice damage thresholds vs basically everything. The problem? It requires decap at max stacks. If there’s anyone with a high cleave or fast firing weapon on your team, you’re basically down a blessing 50% of the game. Flame staff psyker, shreak psyker, and smite psyker all basically disable it entirely. Same with anyone running recon lazgun firing into hordes, and if anyone targets an elite (shot)gunner before you can chop it, you lose all your stacks - which is very easy mind you, given its slow attack speed.
My suggestion is simple: let Decap get a stack if the damage you would have done is greater than or equal to the target’s max hp. This is effectively a oneshot, but without the RNG of teammates’ playstyles or loadouts, and would remove the need to coordinate loadouts around it in havoc (or more accurately looking insane for asking your most damaging class to nerf themself so you can have your +120% finesse).
If this is too broken (i doubt it), you could give it a slight nerf by halfing it’s effect, but doubling its stacks (still +120% max finesse), making it slower to get your stacks while still making it impossible. It’d also be reasonable to have it so you can’t get a stack if you’ve hit a target more than once (cleaving into a shotgunner wouldn’t give you an extra stack if you heavy him after).
I can’t imagine this breaking the game in any signifigant way, it’d just be a nice thing to have, especially considering the current state of the game and how many low-damage/high-area AOE effects exist in the game.