The most crucial thing to know going into the action-thriller Boy Kills World is that it eventually features a phenomenally bloody fight scene — a brutal extended throwdown where faces are smashed, fingers purposefully dig into open wounds, and combatants slowly drag a sharp object through each other’s bodies, ripping through skin and muscle with a tactile squelch. It’s a fight so grueling and brutal that even seasoned action-movie veterans might clench their teeth and mutter in reflexive empathy.