For every thread, someone is the thread killer.
Yes, by definition, someone is the last to post. Occasionally, the thread is a single post -the original post is also the last post. The Original Poster is the Thread Killer.
In fact, more than one person can feel like the thread killer. If I post and no one posts after me for a day, a week, or longer, I'll feel I'm the thread killer and the person who finally posts after me will also feel like the thread killer.
How sad.
Thread starters are "validated" by having someone(s) anyone(s) contribute to their thread.
However, interim posters may only "feel validated" by having someone refer to their specific post by name or quote. If you look at the thread count and realize people are reading the thread, your comments are being read whether they comment or not.
Why do people feel that being the last person to post is killing the thread?
Perhaps readers feel that at that point, everything has now been contributed and new posts will just be repeats. Perhaps that last post has done a masterful job of summarizing and it will be too hard to follow. Perhaps a new topic has come up and interest has shifted. It can also have to do with the time of day you post. Post at 3 am and by the time people get caught up that post can be buried or roll below the page. If your last comment is a "Wow, I agree with everything you said.", there's not a whole lot for anyone to add.
In the email world, I have co-workers who unable to not respond to an email.
B: "Here's the file you requested"
W: "Thanks"
B: "You're Welcome"
W: "That's very kind of you"
B: "Any time. Just let me know"
If you don't "Kill the thread", your in-box will be filled with useless emails. You'll never have time and space to move on to something new.
When threads get to the point that new posts aren't contributing anything new, they die. They die for other valid reasons that are no reflection on the last poster. I guess for some folks there's an underlying fear that if their post was the last, it was judged as lacking in some way, thus the name "thread killer". As you can see, the thread can die a perfectly naturally death unrelated to the last poster.