Or,':S' Is used when someone cannot explain their anger.
OR,it can be used by well-mannered people,at the end of an angry statement,to show that,although they are very angry indeed,they still have enough self control to make a smiley.
OR it can be used to lighten the mood at the end of an angry statement. So that the person who the anger is directed towards realises that the statement is not an invitation to argue.
'I'm sorry, but you're post made me genuinely angry :S'
The person,by adding the ':S' is basically saying,'You've made me angry,but my anger is justifiable. I've made my point,so let's end it there. Don't bother arguing with me.'
Person 2. 'It's just fruit...there's no need to get so angry :S'
Person 1. 'Did you know that The EXAMPLES kicked the lead singer out of the band,just because he was too fat?'
Person 2. 'OH MY GOD! Why the hell did they do that??? Oh my god! That's just ... grrr!'
Person 2. 'I thought you didn't even like The EXAMPLES? Why are you angry?'
Person 1. 'I don't know! I just..grrr! :S'
Person 1. 'I think all people with blonde hair are retarded.'
Person 2. 'That's a really stupid and nasty thing to say.'
Person 1. 'Not really...'
Person 2. 'Yes. It is. It makes me really angry when people say things like that :S'