That's also a possibility, and one that I likeAndor said:I wouldn't put that interpretation on either of those stories, personally. For the 10th plauge I believe the blood on the door was to notify the angel to pass over the house in question. Afterall the Angel of Death is not noted for being big on discrimination and may not have had any other way of differentiating hebrew from egyptian.
As for Lot's wife, simply looking at the angel may have been all it took. He may not have taken the feat that allows you to be selective with your gaze attacks.
So really it's more a matter of Angels not having a magic "Good Guy" sense that lets them instantly know sides, rather than having an agenda that may differ from the god they serve. Or in other words, they can't see those blue circles around the PC's feet.

If the angels were just the extension of God's own will, that wouldn't have been necessary. They would instinctively know who was good and who was evil. Another example of angels being individuals with their own will was the rebellion, where Lucifer and a third of the angels defied God. That wouldn't have happened if they didn't have a will of their own. The remaining angels obey God, but out of their own will.
I think that's what 4e is aiming at, that angels have their own will. They can chose to go against their god if they want to. In the Judeo/Christian/Islam- religions, angels really have nothing to bargain with, since God is almighty. In a D&D-polyhteistic setting, the angels have a better position against their god since that god can be killed or overthrown. Hence the "mercenary"- (very bad choice of word IMO) angle. The angels serve their god, but they don't put up with anything. An angel of vengeance will do vengeance and it will do it in style. It can't be ordered to do a handslapping or delivierance of veiled threats.