Sure, most of them have. Many American presidents served in active duty military prior to becoming president, some heroically. Presumably they killed the enemy themselves or the people in their command killed the enemy. The president is commander-in-chief of the United States military so war time presidents were ultimately responsible for many deaths both American and enemy.
Several presidents served as state governors prior to becoming president. Capital punishment has been the rule rather than the exception in most states for most of American history. The governor signs the death warrant allowing prisoners convicted of capital crimes to be executed.
However, I'll bet the spirit of this question has to do with a more personal type of killing. Two presidents fit this bill. Andrew Jackson challenged a man who had insulted Jackson's wife to a duel and killed him. Grover Cleveland, while serving as sheriff of Erie County New York personally hanged two condemned criminals. He refused to delegate the distasteful task to a subordinate.
