The AI is taking the instant death proc into calculation when it is picking targets (and it has done some weird targeting when wielding it), or it literally can't miss it. I was facing a Brutal AI where every hero they had had a copy of "The Executioner", and they were running around to random targets and instant killing them, breaking pinned if they had to.
For obvious reasons, a weapon with a low chance to trigger instant death, in the hands of an AI who knows what targets it will hit with it, is unfair. And even if the AI does not know, it simply feels cheap.
I lost more units that game to instant death procs by Heroes wielding The Executioner than I lost to every other source in that match combined and the result outright caused me and a friend to abandon the multiplayer game.