You were Spencer's best friend, and he had told you about Maeve because he trusted you. However, that made you a target for the unsub who took Maeve. The unsub was forcing you to kill Maeve... Either you killed her... or the unsub would kill Spencer...

Soon, the unsub led Spencer to the room where they were holding you two

Unsub: "Take off the blindfold. Now."

Spencer took the blindfold off, seeing you holding your gun at Maeve