Nice reversal on the tower defense genre! Sacrificing your fellow monsters for the sake of victory stays true to the spirit of the genre. Also, it's real cool that you managed to squeeze in some intelligent AI!
I would say that the hero might need a wind-up included in their attack animation to give a window for attack or avoidance. It feels like the hero's and monster's hitboxes are the same size, and since the hero usually reacts before I can, the monster always gets hit.