Hmmm... I don't have all of the comics, so I don't know... I know Don for sure is responsible for two major ones (Shredder I, Stockman) and one minor one (an Elite Guard), and Leo offed Shredder II before he became regenerated into a shark mutate... I honestly can't think of any characters killed by the Turtles other than those, though I am sure there are dozens more.
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