I have already posted theory here (Theory #4258) in which I explained why I think that Red John is not a single person.
Today I will post something more important, an essence of this series.

Lets go in the Bruno Hellers mind and motives. 
The name of the series is MENTALIST. Its about mentalism. (Definition: Mentalism is a performing art in which its practitioners, known as mentalists, appear to demonstrate highly developed mental or intuitive abilities.) In this series, Bruno praise a beauty of human mind, a brain skills based on scientific facts.
He often asserts that magic doesnt exist. Jane said that he is not a psychic. Psychics doesnt exist.
Jane abuses his abilities to earn money and he paid a price for that. 
Bruno Haller doesnt like fraud, manipulation and lies. He represents sadists, psychopats and manipulators as a worst kind of human being. According to that, he transfers all those characteristics to the ultimative bad guy - Red John. Also, Bruno Heller has antireligion attitude. He finds wishful thinking naive, bad and stupid. He represents VIsualization as a brain-washing machine, evil organization. 
Bruno Haller praises humanity, moral and forgiveness. 
Patrick Jane will catch Red John only if he would possess all those good characteristics. But, Patrick Jane is not a blissful character. He wants bloody revenge and it brakes him in his persecution of Red John. According to religion theme, it is fight between good and evil. But it is not imaginary fight, it is real fight between human beings. Red John is Devil, and Patrick is still Captain Jerk (anagram of Patrick Jane), and he will catch Red John only when he release his primitive instincts. If Bruno can eliminate revenge in this fight, the goal will be achieved. In that case, Good will win this battle without any stain. 
So, the main point of my theory is that maybe Red John didnt kill his family, or even didnt kill anybody. Red John is  pupeteer, he just pulls all those strings and makes things happen. He is pure evil, source of all bad things, but not an executor! Executors are naive, stupid, small human beings. They made all these crimes, and they are all stimulated by one man, a pure evil. 
He is devil in human body, not an imaginary creature, but real human, evil mentalist.