I'd say that "intent" is enough, but that it needs a lot of qualifiers. For example, someone could be a generally callous person and believe himself easily capable of murder, but find himself unable to do it when the time came. Simultaneously, it's possible for a generally good person to suffer some severe psychological trauma (say, the death of a loved one) and commit murder without really thinking clearly.

My view - in this sort of world of objective good and evil, things can be good or evil without regards to specific actions, if it's in their nature to be that way. A gentle and benevolent person who's been locked in a room his whole life without anyone to be nice to is still a gentle and benevolent person, and a psychopath is still a psychopath even if he hasn't done anything horrible yet.

By way of analogy: a stable bridge is still stable if nothing drives over it, and an unstable one is still unstable even if the wrong combination of events hasn't come up yet. Stability and instability are inherent in the thing itself without regards to past behavior. Also, the stability/instability can change over time, due to reinforcement or sabotage. There's also all sorts of shades of grey, varying degrees of stability from "rock of Gibraltar" to "toothpicks and marshmallows" And only a specialized analysis (in this analogy, "magic") can accurately determine where on the spectrum things lie... though some on the extremes may be more self-evident than others.