In verse 47-51 Jesus talks about their attitudes towards the prophets of the Old Testament. The Pharisees and lawyers were aware that their ancestors had killed many of God's prophets and so they decorated the tombs of the prophets to "show" that they were different to their ancestors, they were better, they were honouring the prophets instead of killing them. But in reality they were just as bad, if not worse, plotting to kill THE prophet, Jesus!
They’ve got the benefit of hindsight, seeing what mistakes their ancestors made, but still they ended up killing Jesus. Religion is not just a misunderstanding, or a mistake - it is a sin, it is God's enemy, it killed God. As John Macarthur said in one of his sermons on this passage: "Religion is not a virtue. It is not man ascending to the highest level of his nobility, it is man descending to the lowest level."