What does Dread Factor do, and what does the Acumen of the King and Princes do?

[Asmodai]

THanks guys

So a High Dread High Acumen Emperor/King is a great thing to have.

I really did not know if High Dread was a vice or a virtue, thanks.

Now that I know I guess I won't stop killing prisoners, unless I capture one of their big shot nobles/royals.

Killing prisoners is generaly good thing. Ransoming other king is provitable(especialy early), but releasing few hundreds of prisoners for about 1500 florins isnt.Killing every enemy soldier is bloody, unfair, and mercilles way of war, but very effective, and should be considered on every occasion. So, dont bother with butcher alike vices and slay every prisoner lower in rank than king. Medieval times were cruel. Iron hand is prefered, if you want keep your kingdom safe.
As for dread and acumen....for governors always choose commanders with high acumen. Acumen rises loyalty in province, and boosts income. Dread is only addition for not having revolts. Obviously, having high dread and acumen is very good thing, but only stats realy important for all game is acumen, command and loyalty.
Piety helps if you are considering crusade or other religious pursuits. Dread for preventing rebellions. But not all factions may crusade, and huge army also helps agains rebels, for preventing or for dealing with rebel rabble.

And when capturing rebels...they are exceptions of the rule above. Dont kill them. Slay only ringleaders or be mercifull and release them all. This boosts loyalty in province and gives your buthers some positive virtues, boosting morale.