While I love Jon and was looking forward to him taking on Ramsay at Winterfell, the actions of Marsh etc are of course entirely justifiable from their POV. The Watch was not supposed to get involved in disputes of the realm, but here they were all but throwing their lot in with Stannis, a defeated usurper. Then Jon was proposing to declare war on the new Warden of the North, or at least his son. Quite right he was, but this was basically revenge, not part of his duties as Lord Commander or a member of the Watch.
has said, Marsh etc weren't privy to Jon's thoughts and must have imagined that this would lead to the Lannisters sending a force up to destroy the remnants of the Nights Watch. These were men who had dedicated their lives to the black. I don't agree with what they did and I hope Jon recovers and nails Ramsay's body to the highest tower in Winterfell, but one can certainly understand that they did what they felt they had to do.