The FBI made sure Bulger remained free for years. Burger had too much dirt on the dirty Boston FBI. After the statute of limitations passed the FBI reluctantly arrested him. Recently, he was ready to testify against Mueller. Big mistake.
Far be it by me, since I know there is no depth too deep for them to stoop, but what concrete information have you seen that he was turning rat?
It is a deliciously conspiratorial theory I must admit.