Ok, the 'swift justive' vice can be gained by killing a certain amount of rebels, the amount is dependant on other factors. This can then lead to other vices, all of which get more dread, some of which have moral penalties (soldiers don't like missing out on their share of loot and ransom money) and happiness vices (people don't like being killed all the time).

The 'mercifull' line is harder to get, requires LOTS of releasing of troops and so on.

THe random justice can be gained by changing what you do often, and can be got for killing the ringleaders. This is only a nuetral option if you always stick to it, and I'm not sure of that.

'Butcher' is got by killing over one thousand prisoner rebels either in or after combat. It gives you +2 dread, and nothing else, but you will get 'scant mercy' or worse if you already have that line of vices, so at least +3 dread in total.

THe happiness boost is not notified, and barely noticable so I tend to kill em all and have despotic rulers, being french we have HUGE acumen, reasonably command and now loads of dread, plus quite a good piety.