Right, which is generally perceived as 'justice', is a loose term. Justice is seen differently in many eyes, thus the differences in our cultures and societies.
However, that's a boring answer. Given time travel, it would be wiser to attempt to alter the course of events of the individual's life before committing to murder. Many of infamous icons such as Hitler underwent psychological torment, or instability in their household before shifting to that genocidal mindset. However, some people simply want the world to suffer.
In the event that one was awarded prediction of the future, I would say it was justified. However, the consequences of executing who held no guilt of the crimes yet to be committed would most likely put you behind bars or death.
So, it simply lies in the hands of the individual. Would you throw your life to waste to prevent damage to a society or a culture?