It is hard to tell from the order of relating the events whether the constraint to kill came before or after Nephi draws Laban's sword. Clearly the narrative places it later, but that makes Nephi's action in drawing the sword fairly curious. Given Nephi's hesitance to kill Laban under the influence of the Spirit, it is highly unlikely that Nephi's first impulse would have been to steal the sword. There must have been some impression from the Spirit which prompted Nephi to draw the sword, and having done so, the more clear command of the Spirit came to him.
It is important to note that the immediate effect of the command of the Spirit is that Nephi struggles with it and attempts to turn from it. Nephi understands that killing Laban falls under his interpretations of the injunction not to kill, and Nephi shrinks from doing so.