Well, if one defines terrorism according to the US State Department, terrorism is:

"Premeditated, politically motivated violence perpetrated against noncombatant* targets by subnational groups or clandestine agents, usually intended to influence an audience."

* This includes civilians and crucially, off duty military personnel.

I'd say there's wiggle room for making a case, of a sort.

Actually that list is far from complete. However the US is also a member of a shrinking but far more despicable club.

Since 2000, only five countries in the world are known to have executed juvenile offenders: China, Democratic Republic of Congo (DRC), Iran, Pakistan, and the United States. Pakistan and China have abolished the juvenile death penalty, but there have been problems in nationwide compliance with the law. - Amnesty International.

I digress.

Pouting aside, the US needs to ask itself how would justice be best served in this case? Dig in its heels and see the man go free (he had committed no crime in Canada, presumably) or accept that US law does not trump Canadian law, see him extradited and punished to the full extent of US law short of execution?

Which scenario do you imagine the relatives of his victim would prefer, and ultimately that's important too, isn't it?