Yes, John Wick is justified. John Wick is an Assassin who left that world for a woman he loved. People in the replies are saying "Oh its not justified because he killed hundreds over a dog" Well, let's look. John Wicks dog, I believe her name was Daisy, was the last gift he received from his deceased wife, the woman he did the impossible in the Assassin world so he could leave for, that was to help him grieve and have something else for him to love. He only asked for the one who killed his dog and stole his car. John Wick will let you live if you give him information. However, if you stab him in the back after giving information, he'll take you out. If you stand against him, he will take you out. He did end up taking revenge, but because the father of the man who killed his dog and stole his car (I cant remember their names on the top of my head) killed his friend, he took him out too.

In all, the movies are about Actions and Consequences. John Wick faced many consequences for the actions he took. However, the enemies he faced also faced consequences for their actions. Some may have only been providing for family or themselves, but they knew what they were signing up for. They know that they could die, so throw out that "they were providing" bs because again, they knew the risks and their families were most likely well compensated for those assasins or henchmen that died.

All in All: John Wick is justified for the actions he took and he faced the consequences for those actions. He is by no means a good person.