The ones for tougher NPC enemy classes should always longer and/or require situational contexts, but there should be finishing moves for all classes. Imagine after knocking its back plating off if you'd been able to assassinate a Hunter, or plant a grenade inside it Forward Unto Dawn style. Now imagine teamwork-style takedowns against the Cabal, with one Guardian ripping off the helmet, so another can plant a headshot, or the first Guardian can do so with their hand cannon. Not quite a yoink but sort of a quicker teamwork takedown. That would make for some great war stories. Assassinations in Halo's PvP and campaign always seemed like these unnecessary things that beat downs made redundant. That's not to say I prefer the instant beat down at all; I'd rather we do away with the single backsmack quick kill and leave in just the finishing moves. Of course the different player v player assassinations could be modified by the weapon being used, too. But my point is that a one-button-kill should always be a trade off with leaving you vulnerable during the animation and not an optional humiliation that accidentally makes you vulnerable.