Four people killed and 32 injured by two teenagers at a 16th birthday party in Alabama
Two people killed at a shooting in a park in Kentucky
Two cheerleaders girls shot at when one went to the wrong car in a car park.
16 year old boy shot by householder when he rang the doorbell at the wrong house when he went to collect his brother.
Four shot including 6 year old when their basketball rolled into neighbours yard
20 year old woman shot when she turned into the wrong driveway
Three students and three adults killed at a school in Nashville.
Mass shootings are loosely defined at those in which four or more people are killed. Most of the very recent ones above don't fall into this category, but were due to mistakes of one sort or another.
Forgetting for a moment all the arguments for and against US gun laws, one has to ask what the heck is wrong with these people.