I would say people naming themselves using the titles of sociopaths or professional criminals, for instance "thug", "hitman", "gangster", etc all fall under terrible.
Not to mention people replacing letters with numbers and people with x's padding their names, especially if their reason is that it's an amazingly overused name, for instance "V3n4m" (Venom).
It just reeks of unoriginality and seems the fastest way to yell at the top of your virtual lungs "I'm 12 years old".
Also, people trying to pretend that they're a soldier, police officer or some other "well-trained badass" is a great way for me to avoid them like they've a tentacle monster with enough STDs to kill Paris Hilton.
So, on the day that I meet "xxXxOf1cc4rV3n4mXx", I can not be held responsible for my actions.