I will do that - thank you for the suggestion.I don't have time to do a numbers comparison, but the points of comparison I'd look at are average 3-round damage output for the Paladin (using a level-appropriate smite spell), Paladin (alpha strike with all the Divine Smite), Battlemaster Fighter (using manuevers), Battlemaster Fighter (using maneuvers and Action Surge) at levels 1, 5, 11, and 20. I'd compare to your Slayer Fighter without & with Action Surge.
I am not publishing this, it is just for my home game so I am not to worried about the narrative aspects of it. However,...As far as specific abilities, I appreciate what you're doing with Slayer's Mien trying to evoke a "menacing warrior" archetype. My personal opinion – not a critique of your mechanics – is that your Slayer falls into the same hyper-focus on mechanics at the expense of narrative that the PHB fighter subclasses fell into.
I think that is a really interesting idea and will take that into consideration.I think you can stay more true to your "menacing warrior" archetype by tweaking this a bit to give this fighter proficiency with Intimidation when they take the subclass, and instead of making it work like a rogue's Reliable Talent, dovetail it into the opportunity attack theme you've got going – Whenever a creature you have successfully intimidated provokes an opportunity attack from you, it is vulnerable to damage you deal to it until the end of its next turn. Effectively adding "soft control" reinforcement to the "menacing warrior" archetype with a bit of mechanical backing to the idea of what being intimidated means.
Yes, I am in agreement as well. I will take another pass at the subclass later today.I also 100% agree with @Stalker0 that the 15th level Deadly Critical doesn't fit your concept. Instead, I wonder if a sort of inverse of the object damage threshold rules could work. I seem to recall there being a 4th edition assassin (or maybe slayer fighter?) that did something like this... where if you reduced an enemy to X or less hit points (it was a small number), then they were reduced to 0 hit points instead. For example, X might be your proficiency modifier (e.g. 3 HP at 5th level) or twice your proficiency modifier (e.g. 6 HP at 5th level). "Deadly Finish" or some such.