There's a possibility verse 23 was not in the original text (it was not in the earliest manuscripts). If it was not "in the original" then Matthew records seven Woes to directed at the Pharisees. This being a book directed at first-century Jews to get them to understand Jesus was the King and Prophet of Old Testament prophecy, it would make sense to me for there to be seven woes. After all, seven is a perfect number in the Jewish faith. But, then, there could be eight woes...
In any case, Jesus sure let loose on these guys! Their problems centered around their hypocrisy.
They claimed to know God's rules, but they actually shut people out of the kingdom of heaven.
They tried hard to convert the Gentiles to the Jewish way of life...only to drag the Gentiles into the Pharisees' hypocrisy.
They made up these intricate rules about how some swearing meant something, but if you swear a certain way it was like crossing your fingers behind your back. They were teaching men how to lie in style.
They made sure they tithed even the smallest herbs from their garden, but were not just, merciful, or faithful.
They made sure they looked good before the people, but Jesus could see the maggots eating their rotten insides. They invited others to come share in their version of righteousness. The people who shared with them became unclean as if they had touched a dead body.
They claim they would not have committed those grievous acts against the prophets. They would have done better than their fathers. But Jesus said they were full partners in their hypocrisy. And they would prove true to His word when they shouted later, "His blood be on our heads and on our children's!"
Ugly, isn't it? Sin is ugly. It takes someone pure to expose evil and show us what it really looks like.