A weed sprout, and the full weed it grows into, are both the same plant. We tend to justify what we consider smaller sins by comparing them to what we could be doing instead. We feel justified in our envy because we are not thieves, in hatred because we are not murderers, and in lust because we are not adulterers.
Sin, whether it be a mere sprout or a fully-grown plant, is still sin in God’s eyes. We, then, should be much more terrified of our wayward thoughts than we are now, and we should be doing a great deal more to put them to death when they arise. Pluck out the weeds before they grow, and do it with the same urgency as if they had already taken over the garden!