I agree, people come here to let off steam all the time and that's fine, expected even. But there's a big difference between coming here and posting about how pissed off you are about the bad day you just had, and using the bad day you just had as an excuse to throw personal insults at people and try to tear them down.
I don't agree with the idea that since these things are said online, they shouldn't make people feel bad. We all know everyone we're talking to is another human being who has the ability to consider our feelings. It'd feel terrible if someone walked up to you on the street and said 'hey, you look like crap today' so why shouldn't you feel bad if you post a picture online and get the same response?
You're completely right Freight, it's up to you how you behave online. Be a dick, be a nice person, it's all under your own control. However, no one can behave like an asshole and then expect people to sympathise with them. If people want to act that way, they can't then complain that no one likes them or treats them well.