Sadly there really isn't a way to state that these men are not doing what they find to be right, following your own 8-fold path, and that the path can be soaked in the blood of others. Buddhism is not a religion inherently awash in "peace" but it is often associated with it as a core ideal simply because of it's take on general violence being unwarranted or unwanted; it does not however "ban" it and just as a combatant can be buddhist so can a pacifist.
The Dali Llama himself has asked the Buddhists of Myanmar to stop their violence against the Rohingya muslims. Unfortunately this sort of killing has been going on for centuries. The Buddhists of this country don't view the Buddhism's peaceful ideals as something that applies to them. It's not like the Rohingya are a threat, they are a minority group in the country. Im sure many people can agree that Buddhism is a peaceful religion, but these people are using violence to terrorize a small group of muslims.