I think the military pacifists here may have missed Max Hasting's point. Yes, better to have no war, but the Great War that Britain couldn't prevent - we simply lacked the military might to do so - would have had disatrous concequences had we not stopped the man with the spiky hat and his troop of butchers.
Ultimately he did not seek war either, rather the supporting of the Austo-Hungarians ended up it occuring.
Princip probably killed someone who was way ahead of his time in seeking to change life in the empire so had he lived Archduke Ferdinand may have altered the make up of Europe significantly.