and why, exactly, should I believe him?
I don’t know. There are those who say that Mossab Hassan Yusuf is a double agent. I’ve had my doubts at time, and might still. But the only way to really know is to listen carefully to all that he says and writes, and notice any inconsistencies. Otherwise, you are stuck trusting nobody who says they defected to the light side of the Force, and killing everybody. “Everybody” is too big a number, and the “defectors” have knowledge that could be useful in combatting the bad guys.
Robert Jordan wrote in his Dragon Reborn series that if you trust everyone, you’re a fool, but if you trust no one, you’re a mad man.