Friar is basically saying that men are both inherently good and evil. It is just like the old saying "The path to Hell is paved in good intentions". Although one may MEAN well, sometimes it ends up not working out in a positive manner. When Romeo is so bent on revenging Mercutio's death by Tybalt, he is indeed doing something admirable. However, the killing of Tybalt sealed his and Juliet's fate in being separated and then their subsequent death.
His speech is a mere foreshadowing of what is to come. The romantic love story between two teenagers doing everything in their power to be together and accepted by their families and it all ends in a huge mess. Just like some of the most beautiful flowers can be deadly to the touch, people can try to do something beautiful and have it end in mere tragedy.