Put them in a between a rock and a hard place, make them choose between two evils, and have someone come after them as a consequence of the lesser one.
Sure they can take out a divine messenger or four... but do they REALLY want to?
One way to approach this is to try adding significant collateral damage to encounters as a way to escalate the conflict:
- do the PCs REALLY want to take out the bad guy(s) if it means killing all those orphans as well?
- if the PCs killed the orphans, how do all the local not-so-bad guys feel about that?
- do the PCs REALLY want to slaughter the demonstrators when the protest turns ugly?
- if the PCs kill all the townsfolk, how does that make the city's patron deity feel?
Just how far are your characters willing to go to get their prize?