Possibly The Lives of Others.
The way I think about Michael Clayton is: assuming Karen is the protagonist, then her main opposition while he's alive is Arthur. So Arthur must be the antagonist. There is a brief period with no antagonist between Arthur's death and when Michael changes sides and accepts the handoff to become antagonist.
In most of the movie before that moment, Michael was trying to convince Arthur to get with the protagonist's and law firm boss's program, so this seems very guardian-like in that world. If the guardian is generally on the protagonist's "side" and the contagonist is generally on the antagonists' "side," then if we only look at the "changing of sides" then Michael goes from "team protag" guardian to "team antag" contag--and it is only the absence of Arthur that de facto makes Michael the antag also. In other words, Michael is Karen's/boss's only real opposition once Michael accepts the baton from Arthur.
Sure, one can offer the facile argument, "Michael is a complex not an archetypal character," but that doesn't answer the questions, "what is Michael's role in the OS before Arthur's death?" and "what is his role in the OS at the end?" Whatever the answers are, it seems like he has 2 very different roles--one early (pro-Karen/boss; anti-Anna) and one at the end (anti-Karen/boss; pro-Anna).