Friends, our Gospel today is the parable of the wheat and the weeds. God sows his good seed, his word, his love and compassion, but his project is met with opposition. And the evil is such that it insinuates itself right into the very fabric of the good.
In classical theology, we speak of evil as a privatio boni, a privation of the good, meaning that evil is always and everywhere parasitic on the good. Just as a parasite is living off of the healthy body (and thereby weakening it), so moral evil lives off of the good soul, the good society, the good Church (and thereby weakens them).
What is the result? That it is exceptionally difficult to extricate the evil from the good without damaging the good. That's why it is extremely difficult—and often counter-productive—to go after these evils with a crusading spirit.
To be sure, there are certain evils that simply have to be addressed—right now, no questions, no hesitations. But there are other evils (and they really are evil) that are best left alone for the time being, lest more damage is done in the process of extricating them.