This is a hard question to answer as is. In the Mormon context, good and evil is decided by God. Or at least that's the the implication of the lesson in killing Laban.

At the same time, though, Mormonism doesn't strictly believe in an omnipotent God, but in a plenipotent God, bound by some greater principles (cf. Alma 42:21-25, nearly contradicted by vs 26). This would seem to support the notion that Mormon God can't just rewrite the rules at will.

That's kind of the deal with Joseph Smith. He was an enigmatic guy. Almost everything he created has an innate ambiguity attached to it because he wasn't above either changing his mind or contradicting himself. That's why I'm not sure it's possible to authoritatively answer this question within the Mormon framework.

From my own point of view, though, if the belief is that there exists a god that is omnipotent, omniscient, and omnibenevolent, the murder of Laban is inescapably evil. Even if we insert free agency into the equation (or is it "moral agency" now?), it should be trivial for that God to engineer a situation where Laban loses access to those plates despite his best efforts without commanding anyone to murder anyone else.