swrushing said:
consider... your paladin and troupe of warriors pass a noble's carriage.
further up ahead, you spy people waiting in ambush. You can sneak up on them.
Start from a paladin's alignment. The lawful thing (depending on the campaign setting, of course, and the fact the paladin
knows this is an ambush set to rob the noble, not a surprise party set by a few misguided villagers

) would be to bring the ambushers to justice. As the local figure of authority is around (the noble), or close-by (liege-lord of the land this happens on), the paladin should be duty-bound to bring those ambushers in alive, for the authorities to judge them
if they have broken a law already by hiding in the bushes. The good thing would be to do this with the least possible loss of life (ideally, no loss).
After these considerations, you can go about this as your "individual code of conduct" allows.
Me personally, I'd handle it the following ways:
a) If my paladin was unsure about what this situation really is about, he'd ask the nobleman if he and his retinue are allowed to join them for a part of the way, or otherwise ride just ahead of them to make clear they are along with the noble. Then he'd take one of his best men along, ride up to the ambushers, hail them and simply call them out, thus ruining their ambush and place the ball in their court. Their reactions would determine my next actions.
b) If my paladin was sure about it being an ambush agains the noble, and not of the "Robin Hood" variety (meaning a BBE nobleman leeching the country dry with disregard to local law and morals, for example), I'd try to place my men in the best tactical position around the ambushers, then call them out and give them one chance to surrender, making it perfectly clear what the consequences of disobeying are, and then go with their reaction.
Remember, just because you should always give them the chance of keeping their lives doesn't mean you shouldn't use a tactical advantage if it increases the chance of your opponent giving up before the fight has started.
Also remember, this is my very personal opinion and take on this example. Basically, even the alignment restrictions of a paladin allow some leeway as long as DM and player are on one sheet of paper where that is concerned.
