"The tares are the sons of the wicked one." Satan has sons too? In John 8:44, Jesus tells those listening to Him, "You are of your father the devil, and the desires of your father you want to do. He was a murderer from the beginning," and so on. Satan has sons!
He not only has sons, he has servants, ministers, even apostles (II Corinthians 11:13-15). This parable says that Satan has secret agents! He has spies, moles, and plants right in the church! We just saw them - ministers, apostles, servants, sons - smack-dab amongst us, and they are so cleverly disguised that we cannot tell the difference between them and true Christians. They are so well disguised that they do not know who they are!
They look converted, talk converted, and seem so pure and righteous oftentimes. Paul tells us in II Corinthians 11:15 that they "transform themselves into ministers of righteousness." They look so good, they say the right things, they serve, and they teach just like the good seed. But they are evil! How subtle Satan is (Genesis 3:1)!
These evil, enemy agents, as good as they look, work to destroy the good seed after the initial period covered in the Parable of the Tares. If Satan fails to get us immediately, he has his plants try to dissuade us from the right way, while we are in church, in our own neighborhoods, when we are feeling relaxed and amongst friends and brethren!
Jesus tells His angels (His servants or messengers) just to leave them there until harvest time. The Bible says elsewhere that those agents help to prove who the truly good seed are (see I Corinthians 11:19; I John 2:19). If we can resist the secret agents, we are doing a pretty good job.