Many terrorist supply ships run under "humanitarian aid" flags.
Secondly, if they were a humanitarian ship, they should have just let them search the ship, find nothing, let Israel say 'ooops, we goofed, sorry, our bad" and just get on with their lives or at least lodge a protest to their respective government officials. The fact that they openly attacked the troops upon boarding only shows me their intentions were less than humanitarian. More like looking for an excuse to start an unnecessary international incident to support a bleeding heart cause.