Free Malakim?

[Archangel Beth]

Meep, Rev Cycle...

What Keeps Angels From Doing Princes' Bidding is... a campaign setting, really. If the contrast is really low, it's more likely that there could be an intersection between a ruthless angel and a sort-of-sympathetic demon (including Princes; Nybbas, Andrealphus, and Valefor take well to Gray-Washing, for instance). If the contrast is higher, then you might get an angel working for a Prince because of blackmail.

Geases are always good for part-time angelic help, if you're a Lilim (or Lilith).

Either way, what keeps the angel from doing this? Probably the Archangel's own internal security measures, and/or Judgment, who tend to figure that any sort of Princely collaboration is a Bad Idea at best, and suitable for execution at worst. It's up to the GM to decide exactly where the lines are drawn, both for the Archangel's own security forces, and for Judgment.

E.g., I have some fic where a very upset Seraph of Judgment actually does some data-entry work for Lilith, in her manor, with part of the deal being that he gets to take notes on the data he's entering. For an angel somewhat in disgrace with Dominic, in service to Destiny and with Yves advocating it mildly for reasons of his own, it was deemed -- In My Campaign -- to be an acceptable risk after Marc looked over the contract and made sure that the Servitor wouldn't come back more festooned with Geases than he started.

(I did mention the "in disgrace" bit, yah? Bad Ideas abounding. *grin*)

Other GMs might have a setting where the Contrast is yoinked up sufficiently that any sort of collaboration is not only stupid as heck, but grounds for execution to put such an insane angel out of its misery.