The image of the martyrdom of St. Sebastian has a lot of imagery that sets the scene. In the picture on the left, St. Sebastian is shown as being completely helpless and vulnerable because he is surrounded by enemies, and in both pictures he is stripped of his clothes, and chained to a tree. Though he has been shot many times with arrows he is still alive and standing showing his resilience and refusal to give up. His enemies are portrayed as horrible men because they continue to attack him, even though he is obviously helpless and they overwhelmingly outnumber him. In the picture on the right there is an image of an angel above him and he is looking up to it and to heaven. This shows his commitment to God. The angel is holding a crown and looks as though it is about to place the crown on St. Sebastian’s head. This makes it seem like St. Sebastian’s martyrdom is destined to happen
With no knowledge of the story of the martyrdom of St. Sebastian and with only the help of the two pictures to reconstruct its events, I would say that St. Sebastian was involved in some sort of battle in which his faith in God was being challenged. He fought against his enemies until all those who stood with him had perished and he was the sole survivor. He refused to give up and renounce his faith, so he was killed by his enemies.