The guy who decided to attack Wain was named Aitor Menendez. He, like many people, had already activated his Soul Altar and killed quite a few monsters. However, he preferred to get stronger in every way possible.
He could increase his power by killing monsters, but he could also get stronger with the help of other people. Unfortunately, he couldn't get a soul or Soul Essence when a human killed a human. Furthermore, a person couldn't steal the souls that another person got from killing monsters.
However, there was no barrier to one person being able to rob another. Just as before the apocalypse, people could rob other people. It was easy, fast, and efficient.
Aitor had not originally planned to attack Wain. After all, Wain was very strong, and Aitor would get nothing if he attacked Wain. But, when he saw the lighter appear in Wain's hands and then disappear and his ring glow simultaneously, Aitor changed his mind. He couldn't pass up this opportunity.
