The friar is gathering herbs and plants that can be beneficial to people, but then he recognizes that the same plants and herbs can be deadly, as well, when used inappropriately. The friar realizes that the same is true for man. Both herbs and man contain the ability for good and evil.
The friar experiences this first hand. In attempting to do something good-agreeing to secretly marry the young lovers, his actions without considering dire consequences lead to Romeo and Juliet's destruction.
This is especially ironic since he cautions Romeo to "Wisely and slow-they stumble that run fast"(II, iii) and he is guilty of ignoring his own advice.
The friar is like a plant or herb, he is in many ways a resource, yet his actions lead to dire consequences for those who rely on him.