When declining an offer, there is almost never anything to be gained from explaining your rationale. Be gracious (i.e. thank the person who delivered the offer to you), indicate that you have decided to pursue another opportunity and move on.
If you feel the need to explain your decision, it can and likely will be misconstrued. "I have decided to accept a competing offer that I feel best fits my current professional and personal goals" may be read as "I don't believe your organization can support my professional and personal goals." If you write "I do not believe that I am a good cultural fit for the organization", somebody is likely to interpret that as "Your culture sucks."
Additionally, it rarely makes sense to notify everybody involved in the interviewing process of your decision not to accept an offer. After your initial interviews, assuming you are still considering the opportunity, you should send a thank you note to the people you met with, as appropriate. Unless you had a relationship with one of these individuals before your interviews, further communication regarding your decision to decline an offer is probably not going to be to your benefit, particularly if you have the hubris to invite the other party "to let you know if you can ever do anything to help them reach their goals." Really?
Finally, consider that your dealings with others in the context of a job search will probably be more enjoyable and well-received if you don't pretend that you have a "personal brand" instead of a reputation.