Pretty sure it's meant to be parsed as "(mass shooting and terrorism) rate": that is, he's claiming that the rate of mass-shootings and the rate of terrorist at

Pretty sure it's meant to be parsed as "(mass shooting and terrorism) rate": that is, he's claiming that the rate of mass-shootings and the rate of terrorist atrocities are both increasing. I don't know whether either part of that is true, but if it's wrong what's wrong isn't that "mass shooting" is singular. (And if, as grandparent claims, suicide rates are "steady [] despite massively increasing spending on it", that does seem like evidence of something getting worse. Again, I don't know whether it's actually true and I wouldn't be surprised if it weren't, but the criticism here doesn't seem fair.)