>> A lot of times rank sitters dont just kill and the end. They multikill.

Multikilling at lower ranks only works against the very newbest. A big reason being, pre-hero your would-be victim will probably level out of your range if you do not level yourself.
Also, from what I see it happens a whole lot more at hero.

Just look at Gzurweeg, he had two of the most feared ragers of his time logging off becuase they were fed up being multikilled by fat ass AP.