I have to tell someone
Why do you feel the need to tell?
Because I think the person should know the truth about me
Depending on what the truth is this may be something you do not want to share. Sometimes it is good to keep our personal things private.
Because I want to be brutally honest.
Well if it is eating you up inside being honest is the best policy. You should make sure you are in a safe space when you tell them if they might have a bad reaction.