Trust me... it's always passive-aggressive.
xD Except when it's possibly an extremely awkward attempt to flirt.
Haha yeah rude is no good. I try to keep it playful. I was once sarcastic with a girl I didn't really know. Later on she actually turned into my longest relationship, but she told me she thought I was a jerk when we first met XD. Oops. ¯\_(ツ)_/¯
Heheheh... I started being more careful about sarcasm when someone took one of my completely sarcastic death threats seriously. Some people have no clue what to do with sarcasm. Especially if you can wield it with the blank face of death.
My reasoning goes that if someone is using sarcasm constantly and it's started to cross the line into rude, three things might be going on: 1. They're simply a jerk in general. 2. They have no idea they've crossed that line of rudeness, which in some cases may be an early indicator of a lack of social awareness/skills/antisociability. 3. They're being passive-aggressive and have something against me but won't deal with me straight up about it. Generally, I fall under category #2 if I start irritating someone. For that reason, I tend to avoid sarcasm that could be misconstrued until I know the person better and they know me better... enough to know that I have no intentions of actually killing them, anyway. >u>