I speculate that Barbaric Blow is bad because the game designers fundamentally just didn't understand how carnage worked. If carnage had the area of effect of PoE1, was capable of critting, and picked up (weapon-based) damage bonuses like a normal attack, maybe it's fine at 2 rage. Instead you get an ability that is essentially a full attack, with a +20% damage on those attacks with a slightly higher chance to crit, and maybe you get like an extra enemy in carnage for 8 extra raw damage. Maybe t