Tragic indeed. This incident will affect us all in more ways than emotional and mental shocks.
Having been passenger on Greyhounds several times I know that it can be a volatile environment after several hours in a not so comfortable seat and chattering passengers who may not have showered in several days, the whine of the engine, crying kids, etc. Not like that on a plane. Security for buses are non-existent in many stations. Anyone can get aboard with ANYTHING, be it knives, guns, whatever! I've personally seen fights break out on buses and seen unruly passengers get thrown off in the middle of nowhere by the drivers (bully for them!!).
I would imagine that we'll be seeing airport type (weapons) screening soon here in the U.S. (if it's not there already -- it's been 3 years since I was on my last Greyhound), possibly a marshal or armed (private) security guard.
You don't expect this type of violence on a bus, but then nobody expected the violence that the 9/11 terrorist committed on the planes before they flew them into the buildings either.
What's sad is that they haven't contacted the mother (probably by now) who is on an enjoyable Alaskan cruise thinking all is right with the world and that her son is coming home. I wouldn't want to be the one to tell her when she disembarks from the ship. How can you console something like that?
Mentally ill? I dare say. Robotic, silent ... something snapped quietly inside the man's head, or the last vestiges of his meds wore off during the bus ride. Will he get the death penalty? Certainly deserves so and is a definite threat to our societies (U.S. and Canadian... and the world for that matter). His victim could've been anyone on the bus. The young man was simply at random. That he's not saying anything allows him the charge of 2nd degree murder because there's nothing that shows premeditation.
All the more reason to learn SD/MA, you just don't know about people.