I realize spouting off about this is like yelling at people that the millennium didn't end in 1999, but instead in 2000. Still, I'm gonna point it out.
There's a difference between avoiding confrontation and being passive-aggressive. Most of what people label passive-aggressive is not actually passive-aggressive. Passive-aggressive is when a person agrees with someone else or quietly acquiesces, avoiding confrontation, but then follows that up by passively resisting what they've agreed to.
Avoiding immediate confrontation by reaming someone via email instead of yelling at them in person? Yeah, that doesn't show a lot of guts, but it's not passive-aggressive. Undermining someone with back-handed compliments out of their earshot? There's a bit of passive-aggressiveness in that. But really, not even so much there. Agreeing to go see the latest Will Ferrell movie you know you'll hate, but then dilly-dallying so that you miss the start time, that's passive-aggressive.
Agreeing with someone about something you hate and then following through on what you agreed to do? Also not passive-aggressive. That's non-confrontational. And it sucks for you cause you got no spine and you are gonna be stuck doing things you don't want to do. But not passive-aggressive.