There is a significant difference between LG anything and an LG paladin, because of 1) the paladin code, and 2) the paladin is probably very religious (for the sake of discussion, no paladins of causes here, if that's possible).
I'd say, break the problem down and ask, how would an LG fighter deal with this situation? Then, how would the paladin code change that answer? Then, how would a highly religious person change that answer?
Also, suppose the angel were just a regular guard, alignment unknown. Would it be unlawful to kill the guard? Would a paladin have explicit legal authority to dispatch the guard (does it even say anywhere whether paladins, in any region of Golarion, get special legal status)? What if the guard were a well-known hero that has saved countless lives? How does changing the guard to a run of the mill angel change the answer from a famous hero? Would a famous angel be even more special than a famous mortal?
This brings up further questions: if a known agent of Good (or at least highly suspected) is guarding treasure that the party needs, and the party needs the treasure for good, maybe there's some misinformation somewhere. Is the party being duped into delivering the MacGuffin to the bad guy, killing an agent of good in the process? Is the guard being duped? If either group is the victim of a trick, what can be done about it? Is the guard being forced to guard the treasure against their will (geas, planar binding), and prevented from being able to converse with the party?
There are a ton of questions! The paladin would attack the angel, though, if it were necessary for self defense, after attempting to flee or maybe even surrender peacefully (no time to argue, something must be done). But the paladin would at least urge the team not to go all-out, try to capture it, or just don't attack once it falls unconscious (and if they disobey, or a major crit foils the KO, that's the others' fault or accidental, but the pally ought to at least pray about that at some point).