Saul was responsible for the murder of Christians but it's unlikely that he did the 'dirty work' himself. His role in persecution was bringing Christians to 'justice' He was much like a law enforcement marshall.
Operating as part of the Roman legal system, he had the authority to aggressively find, arrest, and approve of the execution of Christians. He even went door to door looking for them. This resulted in the death of Steven and other Christians. Acts 8:1-5; Acts 9:1
This is what Paul said AFTER he became a Christian: "My brothers, I AM a Pharisee, the son of a Pharisee." Acts 23:6.
Jesus accused the Pharisees of being self righteous hypocrites. Matthew 23:13; Luke 12:56; Luke 12:15; Mark 7:6;
'Meanwhile, when a crowd of many thousands had gathered, so that they were trampling on one another, Jesus began to speak first to his disciples, saying: "Be on your guard against the yeast of the Pharisees, which is hypocrisy.' Luke 12:1
'And Jesus said to them, "Watch out and beware of the leaven of the Pharisees and Sadducees." ' Matthew 16:6
"How is it that you do not understand that I did not speak to you concerning bread? But beware of the leaven of the Pharisees and Sadducees." Matthew 12:11
With the conversion of the Saul to Christianity, the 'leaven' of the Pharisees influenced the early Christian congregation immediately after Jesus died.
Although the Apostle Paul was no doubt a sincere Christian, it would be virtually impossible for him not to 'color' the teachings of Jesus with the teachings of the Pharisees. Why? Because he spent his entire life as a Pharisee before converting to Christianity.
What the Apostle Paul did would be similar to a Jehovah's Witness becoming a Catholic. It would be almost impossible for such a person to embrace all the teachings of the Catholic Church without being influenced by life long beliefs as a Jehovah's Witness.
Also since Jesus considered the Pharisees to be self righteous hypocrites, it's easy to understand why the teachings of the Apostle Paul deviated from the message of Jesus, the Son of God.
When you compare the teachings of Jesus with the teachings of the Apostle Paul you will notice a difference. What is that difference? It's the 'leaven' of the Pharisees, Jesus warned his followers about.
Jehovah's Witnesses and many other Christian religions assume that 'leaven', pagan practices, and teachings not approved by Jesus worked their way into the early Christian congregation hundreds of years after Jesus died, but this is simply not true.
No matter how zealous the Apostle Paul was as a Christian, when you put a Pharisee in charge of spreading the good news of Jesus, it's easy to see how 'leaven' and 'apostasy' affected the beliefs of Christains almost instantly after Jesus died.
That's why true Christians do not consider the 'entire Bible' as their guide for Christianity. True Christians live their life based on the teachings of Jesus, because only Jesus and God can grant us everlasting life. John 6:67-69