It's not like a goon of a dark guild would kill himself just for laughs. More probably he would be forced to do so by using some curse or other control magic, or he might be an unwilling collaborator being blackmailed. After all, an evil person only believes in his own benefit. He wouldn't be loyal to a guild, he's only loyal to himself. If spilling the beans saves his own life, he couldn't care less about the guild. If he's not personally around anymore, why would he care about the guild's success?
If we are talking about something more profound than a guild, such as a religion worshipping an evil god, then things could be different. After all, a faithful could believe he will be rewarded after death.