By 'personally' I mean being able to physically see them die: not giving the command to bomb a city from afar, or telling a general to send troops somewhere, or things like that.
Did these people write on their experiences, and did it have a marked impact on their rule or their personality?
There have been many presidents that were veterans but it's hard to verify who actually would have seen combat and killed somebody. Not a complete list, but here's a few we can confirmed or reasonably assume:
Andrew Jackson was famous for participating in numerous duels throughout his life and fought in multiple wars
Grover Cleveland was a Sheriff in New York and oversaw multiple executions, it's almost a definite that he personally saw some criminals die.
William McKinley fought in the Civil War as an enlisted man and his regiment saw a great deal of action (Antietam among others)
Teddy Roosevelt participated in the Spanish American War and saw ample combat
George HW Bush was a decorated fighter pilot. While he probably wouldn't have "seen" his targets die, it's a given that he killed his share.
Off the top of my head, some others that saw active service:
George Washington, Monroe (revolutionary) William Henry Harrison (1812) Zachary Taylor (1812, Mexican, others) Franklin Pierce (Mexican) US Grant (Mexican, Civil) B Harrison, Garfield, Hayes (Civil) Eisenhower, Kennedy, Nixon, Ford, Carter (WWII)
Probably leaving a few out