If I was a guilter and had letters from Bamber, I would post extracts of the letters if it supported my view on an issue. You can't have a better source than Bamber himself.
People put up links to witness statements, court testimonies, videos and articles. Both for and against people involved in the case. So I would not feel guilty about putting extracts of letters from Bamber. Other posters have.
Bamber knows his letters could find there way into the public domain, which I'm sure he does not mind, otherwise he would not send them. And him & his supporters dish it out to Julie, the relatives and police enough.