So, instead of properly addressing the issues of mass shootings in the United States, they're doing this; an FPS that trains teachers to deal with school shootings.

It's like they're dancing around the issue of weapon availability in order to militarize the school environment. -_-

Just watched the video. You play as a teacher in the simulation. You have to avoid the shooter and give commands to your students to help keep them safe. I think they only showed the shooter's point of view in part of the video to show how the shooter's AI thinks and works. 

Why not ban weapons instead?

I know this is an old topic and already discussed to death. I also know that reality isn't that simple. I still can't understand why nothing of note is done against gun shootings to date.

Guessing they plan to bring the simulation around to schools on a tour of sorts, and have military personnel play the shooter then. Could be wrong, but that's my guess. 

Now is not the time to talk about gun regulation. There just was a mass shooting. It's so disrespectful to the victims.

I find it hard to think of any reason why letting the teachers play as the shooter would be beneficial. Maybe to get inside the mind of a shooter and try to think how they think, so you can do the opposite and lead your kids to somewhere they won't think to look, or so you can catch them off guard and surprise them? Dunno.