They are simpleminded idiots with a need to satisfy petty needs of winning without honor and skill, but are often too afraid to stand by it. Therefore I ask: Why would they NOT use bullshit nicknames? Hiding their 'true' selves is what they do.
On a semi-related note: Why do top tier players use alternate accounts when practising? Sure, you don't get pm'ed all the time, or, you don't get people absolutely aching to play against you even though they have close to 0% chance of getting a single frag on you.
Why don't they all do like K1llsen? Good-guy K1llsen uses his main account, duels anyone who thinks they are up to the challenge, and while doing so he gains praise from us lowbies who want to be even 1/4 as good as him.