Yingwai Suchaovanich’s letter makes little sense to me. I’m not sure what his message is, other than preaching Muslims’ peaceful nature, and indeed, the great majority of them are.
But the old cliché goes, “Not all Muslims are terrorists, but all the terrorists are Muslims.” One of the pillars of Islamic teaching is the jihad, or holy war, whose ultimate goal is converting the whole of humanity to the one and only true faith. All means are justified to achieve this, including violent ones.
Whether this is real Islam or not is up for debate. But contemporary history is dominated by terrorism in its name.