As always there are different ways to interpret the parable, and some elements in the story can just be discarded, because they are only part of the story's set-up, not its intended meaning. In pretty much all reasonable interpretations of this parable, I think, the tares stand for some kind of thing that is bad or evil. The assertion that the tares were sown by an "enemy" is a part I discard; it's such a blatantly vague dodging of the question of why the tares are there that I take it to mean, "Don't ask why the tares are there—that's not the point of this story." The stuff about the enemy is something the narrator reads, as it were, before the curtain rises; the scene opens on a wheat field with tares.Matthew 13:24.30 wrote:Another parable [Jesus] put forth to them, saying: “The kingdom of heaven is like a man who sowed good seed in his field; but while men slept, his enemy came and sowed tares among the wheat and went his way. But when the grain had sprouted and produced a crop, then the tares also appeared. So the servants of the owner came and said to him, ‘Sir, did you not sow good seed in your field? How then does it have tares?’ He said to them, ‘An enemy has done this.’ The servants said to him, ‘Do you want us then to go and gather them up?’ But he said, ‘No, lest while you gather up the tares you also uproot the wheat with them. Let both grow together until the harvest, and at the time of harvest I will say to the reapers, “First gather together the tares and bind them in bundles to burn them, but gather the wheat into my barn.” ’ ”
So for me the focus of the story is why the tares are tolerated once they are somehow there. The suggestion is that they are too intimately tangled up with good stuff (the wheat) to eliminate them without losing good stuff. So they are tolerated until the good stuff has matured to a point at which it can be distinguished safely from the bad stuff.
As a parable about why bad things persist in the real world, the Parable of the Tares to me is explicitly other-worldly. The goal of the whole exercise of growing stuff is not to keep the field pure while it grows, but to maximise the amount of good wheat that ends up in the barn at the end of the season. So to me the story says that God has something of the luxury of a fiction author who can write drafts, then revise them, tossing out things that don't work and keeping things that do. As an author you can keep an idea going in a draft, to see where it goes and find out if there's anything good hiding in it, knowing that later you can cut out as much as you need, before the final version is done.
It's not so much writing straight with crooked lines as writing for revision.