Initially the series was planned for only 3 seasons. Red John was Brett Partridge in the initial plans. That is why he appears in the first episode stating that "red john comes here". But later scrept was changed and now someone else. No need to take anything series. There are lot of mistakes. For example, compare the two records of Lorelei Martins recorded speech, one in last episode of S5 and one in first episode of S6. In one there is a long pause before Ray Haffner and in the other, no any difference. They will never be able to explain all the issues related to Red John by naming any of the suspects.