Murderers tend to have some axe to grind against a particular person or two - usual motives, love, money, etc. Mass murderers are just crazy, hate all people and just want to kill. Or it is a combination of both - they want to kill one or two specific people and a bunch of others get in the way.
Terrorists on the other hand are motivated by something else. Terrorists hate entire cultures, entire ways of life. A murderer is targeting the people he/she injures or kills. A terrorist is targeting an entire culture - going after the people and the way of life. Their real targets are the people they don't even physically endanger.
Now, which one sounds closer to a couple of radical Islamic kids with all kinds of hatred of America in their wake?
I don’t think we should treat the Boston Bomber as a criminal and I don’t think we should treat the the mass shooters as criminals.
They should all be treated as terrorists.