You're right. I was not imagining that.I think you are not imagining what I am proposing.
My imagined Assassin class would be instant killing only enemies much weaker than them. Basically replicating a popular house rule of autolkilling mooks and minions. You don't bother counting the damage. You look for 6s and if you roll enough
Basically Turn Undead & Destroy Undead but for anything unlimited times. It wouldn't be applicable to strong does. For those you need to do damage.
My combat core of the class would be the Assassin doing pseudo-manevers when they roll 6s. Knockdowns, stuns, blinds etc. Like the monk but rolling well and only attacking once or twice a turn instead of burning all your K and attacking 3-4 timesi. This is in addition to doing a lot of damage. The second aspect would be Assassin getting a buff at the start of combat that disappears after a few turns. Maybe even getting penalties if the fight goes too long.
Out of combat would have a focus on dis guide and mimicking. Possibly good at languages.