Who was the first Christian martyr?
Bonus: What later church leader approved of the execution and stood by while it happened?
Stephen was the first Christian martyr. He was stoned to death and his death set off a wave of persecution. You can read the full account in Acts 6:8 through Acts 8:1.
Acts 8:1 tells us that Saul (Paul) approved of the killing. Paul became an active participant in the persecution of Christians that followed.
Above is a painting of the scene by Adam Elsheimer.