If they could have isolated him on the air bridge, and prevented him from entering the terminal or returning to the plane, they might have been able to avoid shooting him. But I have no idea whether or not it would have been possible for them to do that.

I do think it's a good idea to give law enforcement people training in how to recognize and effectively deal with people with mental illness. I have a relative with bipolar disorder who once was removed from the top of the wing of a airplane (he was standing on top of the airplane wing) in an airport. He has no idea how he got there becuase he was having a psychotic episode at the time.