One of the most respected people in Skyrim if not the very most respected, and pretty much the sole people that are able to teach others the Thu'um in the first place, that's who.
Tell that to the Spartans, who recruited children themselves, and they did so for the purpose of becoming soldiers for contry, not monks of enlightenment and worship. Your looking at that from a modern perspective instead of seeing it how it should be looked at. You still haven't proved btw that they forced him to join.
Yet again, The Imperials themselves are planning to re-war with the Thalmor soon, and I don't see them killing kings to send messages. He didn't have true need. He could have just as easily killed him with sword only anyways.