Alternative Character Interpretation: As the movie develops, it allows room for some human shades of both Aldous Snow and Sarah, who were initially seen as antagonists from Peter's perspective. Not very many break-up movies let the 'bitchy ex-girlfriend' to also be seen as The Woobie, or the other guy to also be a Nice Guy.
Draco in Leather Pants: Sarah Marshall, despite being portrayed as a selfish, petty and genuinely unpleasant individual, will still have audience members defend her to the death...presumably because she's played by the very attractive Kristen Bell.
Peter starts out as this. He just can't get over Sarah and goes to Hawaii to try and forget her only to find her staying at the same hotel. He gets better, though.
Even Rachel has a moment when Peter confesses he tried to sleep with Sarah again. A lot of tourists probably try to use her as a vacation fling, and at that point it looks as though Peter was doing just that.