What does Acts 7:52 mean?

Stephen is bringing his argument home. His accusers claim to defend the Mosaic law, but they don't even have the hearts to understand it. They actively resist the leading of the Holy Spirit (Acts 7:51), just as their forefathers did throughout the Old Testament. Now, Stephen declares the biggest irony of all. These same forefathers, whose legacy they claim to honor, persecuted the prophets God sent them to turn their hearts back to Him. Jezebel killed many prophets (1 Kings 19:10). God sent more, warning of the Babylonian captivity, and the people mocked them (2 Chronicles 36:15–16). Jesus accused the scribes and Pharisees—Jewish religious leaders—of continuing that legacy (Luke 11:47–48), a legacy Stephen's audience continues as well.

The ironic part is that those prophets often spoke about the Messiah. It's thought that the prophet described as "sawn in two" (Hebrews 11:37) was Isaiah, one of the most prolific writers about the coming Messiah. If Stephen's audience had studied and accepted the prophets with hearts bent toward God and focused on understanding, they would have recognized that Jesus is the Messiah (Luke 24:26–27; John 5:39–40). Instead, they functionally "killed" the prophets by disregarding their words. Not only that, they had actually conspired to have Jesus crucified; they "betrayed and murdered" the Messiah, the Righteous One.

Verses 51 through 53 feel rather abrupt, compared to Stephen's prior words. It's possible that Stephen senses the crowd is getting agitated and he must cut his speech short. It's also possible he realizes he is about to become one of those whom the "sons" will persecute and kill.