Well, he doesn't use his magic on purpose. Virtue and Sin don't even know how their magic even works, it is purely based on their personality, desires, and emotions. Sin was going to use his magic on the fact that he was just really angry at everyone yelling at him and looking like basically everyone wanted to fight him. Also, he really did not like Sasha at the moment and wanted her to shut up which actually would have been the breaking point until someone bashed him in the face with a shield. So basically, he was going to accidentally use his magic on everyone in the guild because he was angry at everyone since they did not treat him very well, or at least talk to him in a reasonable and peaceful manner.