Ah, so he'd have to be alone in a region where we have no mobile troops or militia. Then if one of our nobles moved there with murderous settings it would trigger the battle? If that's right it probably explains it. So, all in all, it's fairly unlikely that a battle will happen when you're set on murderous unless you can create a very specific set of circumstances (the target noble is alone in a region when you move there set on murderous).
Yes. Because murderous only makes you "attack" against other realms, it won't force them to "attack" your defending units.
If a foreign noble travels through your region, he won't have "control of the battlefield", and therefore "defender" status, unless he sits in the region alone (or with only other foreigners). If you have militia there, or realm mates, he'd need to defeat them first to gain "control of the battlefield", and as he doesn't, these units thereby confer control and "defender" status to their peers that arrive later.
The result is that using murderous settings to trigger battles in your own lands is rather difficult, but much easier to achieve in foreign lands.