Bree doesn't want to confide in Renee because she doesn't consider them to be friends. That's not going to stop Renee from keeping an eye on her. She doesn't want to relive the pain of watching someone commit suicide like her mom did. Bree knows that Renee saved her life. She says, "I realize that makes you the best friend I'll ever have."
Bree and Lynette make a chart of people who know about the Alejandro situation to help them determine who may have killed Chuck. It could have been the person who wrote the initial letter. And it could be an unknown figure represented by the big red question mark on their chart. That's probably the same person who just left another note in Bree's mailbox with two little words, "You're Welcome."