Here's a fairly in-depth post on this subject https://www.reddit.com/r/AskHistorians/comments/1mpx0p/which_us_presidents_have_killed_someone_directly/?ref=search_posts

It's an interesting question, and one we don't have a definitive answer to. A lot of presidents served as junior officers at one point or another. Most of them fired their weapons, what we don't know is how often they connected.

Here are some certainties.

• Andrew Jackson: Jackson engaged in duels and served in multiple wars. At the very least he killed Charles Dickinson in a duel on May 30, 1806 after Dickinson claimed Jackson had welshed on a bet, and insulted Jackson's wife.
• Grover Cleveland: As the Sheriff in Buffalo, Cleveland personally acted as hangman for at least two executions. The first on September 6, 1872, Patrick Morrissey. The second on February 14, 1873, John Gaffney.
• Theodore Roosevelt: On July 1st, 1898 he shot an unknown Spanish soldier on Kettle Hill as part of the Battle for San Juan Hill.

Here are some maybes. All of the following served as junior military officers. All fired weapons. I don't know if we have any confirmed kills. George Washington, James Monroe, William Henry Harrison, Zachary Taylor, Franklin Pierce, Ulysses S. Grant, Rutherford B. Hayes, James Garfield, Benjamin Harrison, William McKinley, Harry Truman, John F. Kennedy, Gerald Ford, George H. W. Bush. Some of these are more likely than others. Truman for example was an artillery officer and apparently liked to personally fire the gun. It is very likely that he killed someone. Likewise Bush flew 58 combat missions it is very likely that at least one of his bombs found a target.

There are other presidents who served in the military. Lincoln was a captain in the Blackhawk War. But they don't appear to have seen combat. So the above list of seventeen include the likely candidates.