I agree with you that the dynamism and possible win conditions in 6v6 solo ranked are different than those in 6v6 group premade. It's true that in 6v6 group premades you'll have melee trains, in which Crimson Death is head and shoulders above any other mastery-specific ability. It's also true that the premades sit in discord and are able to coordinate and punish you for every possible mistake (like the Soul Killer trip). In a 6v6 solo where you play with 5 unorgaized solo players against 6 unorganized solo players the realities are different.
That's exactly why I was arguing about things like Soul Killer, Unstoppable Fury, Choking Fury, even Bolstering Anger. The different settings of 6v6 solo can make certain abilities, within certain conditions, useful. If it was a question of 6v6 premade, then there's only really one build, the one adamska/azeyune posted. I also believe that build is absolutely fine for 6v6 solo, but perhaps you can change it around a little bit depending on your opposition. Also, I think if you want Soul Killer, you shouldn't leave Crimson Death out, but make a build where both are present. That'll be harder the lower you renown rank is, however it is doable in any 40+ setting.
Merry Christmas!