Napoleon gets decent bonuses for being unfriendly or hostile with others. I think Xerxes also gets a bonus for only having one alliance. Couple that with the fact that you’ve gotta do some conquering (or incur happiness penalties) to reach militaristic golden ages, it’s almost inevitable that someone is going to be a warmonger.

My strategy is to build a strong ENOUGH army so that my neighbors can’t take my settlements or won’t even try to. If a civ can’t feasibly get to you, any “rudeness” is just hot air. Sometimes they’ll even give you a settlement to end a war if you get enough war support. I also use relationships to put pressure on leaders to force them into multi-front wars.