In the latter part of his career, Isaac Newton was the Master of the Mint, an important (and well-paying) position within the government, and his primary preoccupation as such was improving the status of English coinage (which had in recent memory been terribly debased), and this including tracking down counterfeiters. The punishment for counterfeiting was death by hanging, and Newton himself acted as a sort of detective and on his evidence, several men were sent to the gallows. Newton used the threat of death as a way to get some (not all) of his suspects to confess and name collaborators.

I have not heard of Newton himself killing anyone by his own hands, however. I find it very unlikely on the face of it.

One can read all about Newton's interesting career at the mint in Thomas Levenson's Newton and the Counterfeiter. It's a good read.