To a large extent, whether or not Shylock is justified in seeking revenge or not is up to each individual reader. I am sure that there are plenty of people out there that believe Shylock's desire for revenge is completely justified. Those people believe in an eye for an eye. Antonio is undercutting Shylock's profit margins by not charging interest. Antonio has publicly insulted Shylock and even spit on him. Finally, Antonio and other Christians segregate Shylock. It certainly appears that Shylock has a lot of reasons to dislike Antonio and want to seek some of kind revenge.
Personally though, I don't think he is justified. I personally don't see revenge ever being justified. Additionally, I just don't see the point, because I don't think it would solve anything for Shylock. He might feel better initially, but Antonio is still going to undercut his business and bad mouth him in public. If Shylock sought revenge, the antagonism between the two men would probably escalate. I just don't feel that vengeance best serves Shylock.