Virtue Itself Turns Vice Being Misapplied
What does this quote from Romeo and Juliet mean? "Virtue itself turns vice, being misapllied,/A vice sometime by action dignified."
In this quote, the friar is saying that good and bad qualities may not be as cut and dried as might be assumed, and that good can turn into bad and vice versa.
The friar is ruminating at this point on the properties of the various plants and herbs he is gathering, observing that a plant not yet fully grown has the potential for both good and bad: 'Within the infant rind of this weak flower/ poison hath residence, and medicine power'. He applies this to human beings also. In essence what he is saying is that living things all have the potential for good and also harm. It really depends on how these qualities are used. If virtue is 'misapplied', that is to say used wrongly, it becomes harmful. Conversely, vice might blossom into something good, given the right circumstances, as a previous answer stated.
As observed in another answer, this analogy can be applied to Romeo and Juliet. Their love is a sweet and positive thing to begin with, all the more so when contrasted with the long-standing hostility between their respective families. However, they let their love dominate them too much; they end up with a surfeit of it.
The human tendency to intemperate emotion (seen in both the love of Romeo and Juliet and the hatred that otherwise prevails between the Montagues and Capulets) is probably what the friar also has in mind when referring to a flower:
For this, being smelt, with that part cheers each part;
Being tasted, slays all senses with the heart.
The friar is hinting here that it is all right to partake a little of something, but to indulge it too much can be overwhelming and even fatal, as the young lovers' passion proves to be. When they only have a whiff of love, it brings them happiness, but once they actually taste it, indulge it, it becomes too much and overcomes them; it 'slays all senses'.
In this quote, the Friar asserts that vice and virtue are not necessarily black and white concepts, and something that is a vice in one circumstance might turn out to be a virtue in another. This suggestion has significant importance for the play as a whole.
Romeo and Juliet, for instance, are madly in love with one another. It goes without saying that generally, most people consider love to be good, possibly even virtuous, depending on who you talk to. However, Romeo and Juliet's love for one another ultimately drives them to committing suicide on each other's behalf. Likewise, while the advice of parents is often presented as wise and beneficial, in the play the disastrous opinions of Romeo and Juliet's families is ultimately the main force that pushes the young lovers into exile. As such, it is clear that, just as the Friar predicted, vice and virtue are not set in stone, and complex contexts can transform virtue into vice.
The Friar is saying that all things have the potential for both help and for harm (virtue and vice). His soliloquy is while he is picking herbs for his potions, but the properties do not relate strictly to plants. No matter what good qualities something (or someone) possesses, if they are misused for any reason, the result may be more harmful than helpful. You've heard of "too much of a good thing...", right? This idea will fit both Romeo and Juliet as the play progesses (foreshadowing) because, for them, too much love, no matter how wonderful it may be, once it is "misapplied" (becomes obsessive) will turn out to be disastrous. For them, "virtue itself turns vice" and they both commit suicide for love.