Read Luke 11:48 Using Other Translations
Truly ye bear witness that ye allow the deeds of your fathers: for they indeed killed them, and ye build their sepulchres.
So you are witnesses and you consent to the deeds of your fathers, for they killed them, and you build their tombs.
But in fact, you stand as witnesses who agree with what your ancestors did. They killed the prophets, and you join in their crime by building the monuments!