Though he was a horrible president (do some research), the assassination was not justified, nor is any assassination attempt justified, except during war. This, obviously, was after the war, so, therefore, it was not justified. A lot of people consider Booth to be evil. In my opinion, he is not evil, but he is no hero either. He committed a horrible crime, but with good intentions.
No assassination of anyone is justified, especially that of president Abraham Lincoln. All people are to be viewed as special and all life is valuable and by assassinating someone you are stating that they are not, which he as a president was. He made a lot of enemies and it is very easy to understand why they might have wanted him dead, but assassinating him was not the answer.
There is absolutely no way that the assassination of Abraham Lincoln can be considered justified. Killing someone because you didn't get what you wanted is never justified. Neither were the deaths of more than six hundred thousand soldiers on both sides of the Civil War conflict that preceded his death.