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Mankind has been killing things with fire since our early ancestors, so if something undesireable needs to be done away with, fire is a tried and true way. Now the term “kill it with fire” or often “KILL IT WITH FIRE” seems to have peaked once in 2007, before finally gaining a foothold online in 2008. according to google trends.
The earliest instance we could find was here in a comments section on an article about furries, but whether this is the first instance or just someone posting what they saw on another website is unknown.
“Kill it with fire” is believed to be a reference to the 1982 John Carpenter classic The Thing.
The term “kill it with fire” is used to respond to a post where something is so disgusting that the only emotion in your complete being is to burn that motherf*cker down. The term is also placed on pictures with flamethrowers and their ilk to show how much fire should be used in the situation at hand.