Good subject!
I have become more convinced with time that first impressions can be very misleading.
This holds true for an in-person encounter, work-only encounter or on-line encounter.
The old cliche is true "you don't really know someone unless you live with him/her."
Some people who post on JWD or other sites can put on an online persona that is not true reflection of their overall disposition in real life.
Your other subject may well be true: Some who appear "nice" are hiding their evil side.
It takes several years of close association to really be able to say "I KNOW that person" and even then they may surprise you by doing something you did not initially think they would be capable of doing.
YC