About knocking out mercenaries

Hi guys, just started my first NG+ at lvl 99. As all of ship lieutenants are gone, I was trying to recruit new lieutenants. I'm playing on Nightmare, and build has assassin damage 540k+, 100% crit chance with 715% crit damage at fh. I just did an unarmed strike to a mercenary, and to my surprise it instantly emptied all of his hp and knocked him out. After this I did this on another merc who takes less damage from assassinations, and again the same thing happened lol. I'm wondering whether this is a glitch or my build is actually too OP?