This is what Urban Dictionary has to say about it............ "QUOTE"
thread killer
A person who makes some kind of awkward comment on a status thread, which stops the thread dead in it's tracks.
Some of the reasons for the thread stopping on the thread killer's comment:
1. Other threaders don't know this person well, so take the comment in a negative light.
2. They remember her/him being a "weirdo" in school/former or current job, etc.
3. Other comments in the past by the thread killer were confusing and possibly: crude, inappropriate or might appear hurtful, due to the absence of a smilies or winkies.
Look at how long this badminton status thread was going, until Brian commented this:
The best strategy is to try to hit the shuttlecock straight into the opponent's naughty places.
He sure is a thread killer! "UNQUOTE"