Why did Jack Ruby killed lee harvey oswald?

Answer 1

Ruby himself claimed that he did it to spare Jackie pain and suffering.

Others have suggested that Ruby acted out of anger or rage, as has occurred in many cases of 'vigilante justice' even in modern times.

Still others suggest that Ruby was part of a conspiracy, one that included killing President Kennedy and then killing his apparent assassin. Under that theory, Oswald's claim that he was a 'patsy' would be true, although ample evidence exists that Oswald was indeed guilty, including his separate murder of Dallas policeman J. D. Tippit.

Answer 2

Jack Ruby died before he got to trial, so there is no known reason, only speculation.