Even the worst serial killers can be stopped if everyone gives them puppy dog eyes.
Meet sociopaths.
He was being sarcastic.
EDIT: Violence is the part of conflict that happens when diplomacy fails. If everyone agreed to use diplomacy and not violence, and even when they weren't getting what they want to refuse to turn to violence, that would stop warfare. But there's bound to be people who are diplomatically powerless, economically powerless, etc. and for whom the choice is not "a good life if I win, or a shitty life if I lose" but survival vs. death. It makes sense for those people to fight. The people who would say they should just lay down and die rather than turn to violence are probably coming at it from a diplomatically- or economically-powerful position. Of
course the ruling class would want the underclass to die rather than fight.
There are also going to be situations where the status quo is unacceptable to one side or the other. Which is to say, if diplomacy fails and no change can be agreed upon, one side is able to accept the status quo but the other considers that vile and impossible. A has no compelling interest to give anything up to B, because A is fine with no agreement. B can't afford to lose, and can't afford to have no agreement. Which means B must give and give until they butter A up enough to form an agreement. The minimal terms for A to say yes will probably be so unappealing to B that they would consider it tantamount to no agreement - or even worse, an agreed loss from the status quo. B is diplomatically powerless. Why should B even consider diplomacy worthwhile if it's impossible for B to operate in that arena? If you have no money but lots of goats, you will engage in a trade economy and not even consider a money transaction as an option.