The lion is the king of beasts. To lionize someone is to see them as important as a lion. Republicans continue to lionize Ronald Reagan as their ultimate hero.
The lion is a symbol of pride, strength, courage, and nobility, so when you lionize someone, you attribute to them all those marvelous traits. Many people are uncomfortable with being lionized, since being made to seem important comes with responsibilities that are sometimes a burden. After the shooting, the bystander resisted people's efforts to lionize him. He was only doing what any person would do, he protested. "Don't make me into a hero. I don't want to be lionized."