Well, as a matter of fact, it is sad that "the Kingdom of Heaven" (all the churches until the Rapture) will be corrupted. But that is exactly what the Lord was teaching in the parables of the Kingdom. Please note that out of the seven parables given in Matthew 13, there are only two which do not speak of the influence of Satan within the churches, or the presence of false Christians. Those are (a) the treasure in the field and (b) the Pearl of Great Price. All the others show how Christendom (the outward manifestation of the Kingdom of Heaven) is affected by Satan and evil spirits.
Satan at work, the Gospel attacked
When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side.
Satan at work, false Christians present
But while men slept, his enemy came and sowed tares among the wheat, and went his way...The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels.
Evil spirits at work (the birds of the air)
Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
Satan at work, churches corrupted
Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.
False Christians present, cast into Hell
Which, when it was full, they drew to shore, and sat down, and gathered the good into vessels, but cast the bad away.
As to the evil meaning of leaven please note (Mt 16:6,11,12; Mk 8:15; Lk 12:1; 1 Cor 5:6-8; Gal 5:6-9):
Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees...How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees? Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.
Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us: Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.
For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love. Ye did run well; who did hinder you that ye should not obey the truth? This persuasion cometh not of him that calleth you. A little leaven leaveneth the whole lump.