Murderous settings are unreliable because of one very important distinction: You cannot be a "murderous defender" and force someone else to attack you. This is related to the explanation in the other thread about "possible diplomacy bug". The game first decides who the defender is, then looks for anyone that wants to fight them. If the one with the Murderous settings is picked as the Defender, then it's up to the other person's settings as to whether or not there is a battle. The Defender's encounter settings are never checked!
What you have to do is force the foreigners to be the Defender, so that their encounter settings are not used to determine if there will be a battle. This is not easy. When you're wandering through someone else's region, the region owner will almost always be the Defender. You have to jump through some hoops to make the foreigner the Defender, and even then the foreigner usually has to make a mistake.
Remember:
Step one of the battle system is "Who's the defender?". In this case, the region owner in their own region, with no TO running, is almost always the defender.
Step two is "Who wants to fight them?" If the foreigner is neutral or defensive, then they don't want to fight. So no battle.
How do you make the foreigner the defender? I can think of two ways.
The first way requires the invader to be stationary, while no home troops are in the region. If the foreign troops are in the region across a turn change, and all the home region troops are marked as "freshly arrived", then the foreigners will control the field and be defenders. The region owner is now a potential attacker, and their Murderous encounter settings will trigger a battle. If the foreign troops keep moving, this method won't work. The foreigners will always be "freshly arrived", and never the defender.
The second way is to arrange it so that the home troops and the foreigners arrive in the region at the same time, and for the home troops to have a *smaller* force. If even one home realm unit is already in the region when everyone else arrives, even if it is a militia unit or peasant group, then the home team will control the field and be the defenders. All home realm troops have to be freshly arrived. The reason for the home team being smaller is that when all other means of determining the defender have failed, the battle system defaults to "Bigger army is the defender". And if all units are freshly arrived, and there's no TO going, and there are no peasant mobs, then the game throws up its hands in disgust and picks the biggest army.
Both of these methods are very difficult to arrange. There will usually end up being some militia unit already there, or a rogue monster group popping up to throw things off. Or you have to chase the invader and can't ever manage to arrive in the same region together. And even if you do, you'll have to have a smaller force to trigger the battle, which means you'll probably lose.
As always, the real answer is to fix your screwed up diplomacy. If you want to fight them, declare war.
In Suville/Caergoth's case the two realms weren't willing to do that, because it would mean canceling their only alliance and being cut loose to fight by themselves. In Suville's case, it was made clear that if they dropped their alliance with Strombran, that Strombran would walk away, and Suville would have to face Tara and Caergoth on their own. I imagine that Caergoth got the same answer from Tara. Strombran wasn't willing to break out of the federation, and neither was Tara. Both realms were more than happy to sit pretty and be invulnerable, while having all the fun they wanted burning down regions of realms that couldn't possibly fight back.
Strombran could have gotten away with it if they hadn't gotten greedy. Who knows how long Suville would have put up with it, if it hadn't been for the Wynford debacle, and the revelation that they claimed Riverholm for themselves.
