Their deaths were proof positive that they believed what had happened to them with regard to Christ’s resurrection. They would not have suffered and died for a known fallacy or scheme.
They became energized when they were able to see the proof of resurrection. When the disciples saw the resurrected Jesus, they became bold in their defense of Christ. This boldness led to them die as martyrs for their beliefs.
Jesus followers became bold after Jesus resurrection. Their doubt was taken away and now they knew what Jesus had told them was true. Now they knew there was an afterlife and this life was temporary. So they followed Jesus’ instruction to spread his word to every nation and people.