What is/are the factor(s) that distinguish homicide from terrorism?

Acting to further a political agenda is the difference.

Terrorism: The unlawful use or threatened use of force or violence by a person or an organized group against people or property with the intention of intimidating or coercing societies or governments, often for ideological or political reasons.

Huge grey area there....since that's a language definition, not a legal definition.

Of course Hasan is both a terrorist and a mass murderer. But it's not obvious yet, if he's a lone terrorist, or part of a larger organization.

A person acting alone can commit acts of terrorism, no doubt. However, I think the defining piece of the definition is that they are acting out of an ideological or political purpose, in an attempt to coerce society.

If someone goes on a shooting spree, killing people indiscriminately, then they are a mass murderer. If they target a group of people during that killing spree, motivated by ideology or politics...then they are terrorists.

Seems to me Hasan could easily fall under the "enemy combatant" category, since he attacked non-civilians.

Let's also not forget that he will likey be charged, tried, and convicted swiftly in a military court.... where the only sentence he's likely to get is the death penalty.

I think something should only be called terrorism if the person or organization that committed the act intended to invoke terror. I don't believe it has to involve murder. Kidnapping and highjacking, even without any resulting death, can be forms of terrorism, for example.