a) If you define a thread killer as a person whose post ends the discussion, then offering a solution to a problem certainly qualifies as thread killing.
Plenty of thread killers out there?
Yet I believe this is not what this thread is about.
b) If someone can kill a thread by posting rubbish, then that was a stupid thread anyway. Otherwise, rubbish would be ignored and a discussion would continue until a solution is found.
So I still believe nobody can stop a meaningful discussion (i.e. kill a thread), without offering a solution or reaching consensus. Hence, thread killers do not exist.