I chose my SN as it said sth about me, and how i felt about myself (& still do to a fair degree). Sometimes when i post i kind of wish i had a different SN, as might affect how people respond to what i put in that post (eg a more positive SN for an uplifting-type post). Other times my SN couldn't be more perfect for what i want to post. But now that i've been here a while now, and i think may be fairly well recognised, i think that my somewhat-negative SN doesn't affect that much how people repond (or expect to respond) to my posts. And also it mightn't affect how they view/see me now. With the newer users however, well maybe they might be more affected by my SN, and consequently might not "trust" me as much - ie may create a (possibly somewhat negative) "vision" or "judgement" of how they think i am as a "virtual person" (due to the "first impression" of my SN/avatar/sig/etc), and have already formed an opinion of what the contents of my posts will be even before reading a single line of such a post.
At times I have considered changing it (eg to "tsl"), but the problem is i'm kind of "attached" to it now (it's part of my "identity" on here, and others "know" me by this SN). And yes, maybe part of it is my SN name makes me feel "secure with who i am" on here, and so i can be confident about what sorts of things i can post (as i know "who i am" or what my "role" is); which could be another reason why i'm attached to it.
But having a negative-type SN in this forum doesn't necessarily mean you "take on that persona" in everything you do in life - you don't (well i don't). On all of the sites on the web i belong to, i usu have a different SN (eg (not actual) "that_computer_guy", "IWannaLearn", "Prinze", "aBitDifferent", "FredDIY"). It all depends on the context (site).
So, some of my SN's are a bit negative, some are positive, a few are lightly-humourous, and most are just neutral. So in each of those sites i frequent, although i bring with me my current mood & life issues, i tend to focus on the task at hand, and usually am feeling quite good whilst doing so. And maybe my SN does affect my mindset a little when i'm doing my stuff on a site - i dunno. But i know that when i first chose that SN, my mindset at that time had a "say" in it, and also my usual mindsets i have daily, personality, & past experiences. Also a bit of logic (eg if site is about computers, and i know a fair bit about them --> SN = "that_computer_guy")
But i do get what you're saying Entourage!. Having a negative SN (even in just one site, but which you visit regulalry) probably does help to maintain the "victim mentality" one has. You might not feel it affects you consciously (as "used" to it, and is associated with many of your various posts, of which a fair few might be "victim-like", but others are not). But unconsciously it is another matter entirely (the "rules" are different!)