To be very accurate the passage in question (Luke 11:50), and it's declaration by Jesus, is directed to the lawyers and not the Pharisees. They had felt included in Jesus' reproach of the Pharisees and when one of them pointed it out Jesus did include them and proclaimed for them woes of their own:
Then answered one of the lawyers, and said unto him, Master, thus saying thou reproachest us also. - Luke 11:45
The particular specialty of the lawyers was just that; study, dissemination, and application of Jewish law by which it should have been manifest that the killing of the prophets and righteous men by their forebears was egregiously wrong. Instead of perceiving this from the law and repenting in sackcloth and ashes they treated them to the lip service and accolades of sepulchers. We are to understand from the context (and the word mnemeion (sepulcher)) that these tombs should have at least been taught as a reminder of their father's wrongful treatment of these men if not even designed to remind, as by inscription, for example. In this way they were preventing accurate memorialization of these holy men of old and tacitly agreeing with their father's treatment of them by neglecting to remember the sin. Revisionist history is probably not all that new.
We know that within "this generation" Jesus has already begun to call and to find His lost sheep: There are already Apostles and disciples. Individuals are responding to the Gospel of the Kingdom and are being saved from within "this generation" so "this generation" cannot refer to each and every individual human alive at that time. This makes the noun genea (generation) here more likely to refer to a period of time itself than all the individuals within that period of time. Indeed, this is how the word is predominantly used in the NT.
The word rendered "required" is the verb εξζητεω (exzeteo), meaning to seek out, implying an unraveling or intended retrieval of something from the environment it was found in. It is not that God is intending to bleed each individual alive at the time but that the blood of these righteous men is crying out for justice and God has coming at this time seeking it out.
So we find that it is during this time period (genea) that the blood of all the prophets, which was shed from the foundation of the world, is required (that is to say, sought out) for justice. All of the Law and Prophets has testified of this coming King, this time of judgement, and in this genea, this time period, amongst all manner of confirmatory signs, He was not only outright opposed by leadership but opposition was taught and encouraged.
God is not unjust. Those who have received Christ are not sought out for justice during this genea for it is Christ's blood that is shed for them instead. The woes are not for every single individual but for those who possess enough information to know the time of their visitation and refuse it, teaching others to do so as well.