I mean it fires pepper spray bombs. Even when it does its job correctly, and hits an assailant, it is a wide area weapon that takes time to dissipate, particularly in an enclosed space. It will almost certainly effect other people to some degree, including those fleeing.

I don't have a problem with pepper bombs being autonomously fired on an armed assailant, in itself, by which I mean it's better than an autonomous pistol firing slugs into "assailants". I would even accept some minimized-as-much-as-possible risk that it incorrectly targets the wrong person with pepper spray if they are not in immediate danger. The problem will be when the pepper bombs slow down those fleeing or divert them towards the gunman or leave someone incapacitated and unable to flee or sense danger, and hinder emergency response teams, school staff, or other individuals trying to rescue kids. A clever gunman with a gas mask could even use the drones to corral victims into easy targets. It's just not a reliable fix to the problem and creates opportunities for whole new problems.