Anytime a person or people attack another individual it makes the situation much worse. Racial slurs are a horrible form of discrimination and expression of hate, but violence is not the way to end discrimination. There are legal actions that can be taken or the individual can be exposed on social media to bring social pressure on them to stop their behavior.
No, we can never excuse physical violence as a response to verbal exchanges. That opens up to a slippery slope that is completely incompatible with the western legal system. Who decides what provocative words are strong enough to warrant a punch? What if one judge deems a certain slur valid, while another disagrees and charges the insulted party with assault? It's a subjective can of worms that should never be opened.
To show report for Trump while also using a racial slur can be viewed as inciting violence and is certainly ground for a strong response but actually using physical violence reduces an individual to an even a lower level than the verbal assailant. It is far more effective to fight words with words or simply maintain a dignified silence.
No, violence is not justified when someone makes a comment that you do not like even if it is a racial slur. People should not say things that they know are hurtful and offensive to others, however, when they do they still should not be victims of attacks. It is important to obey the law, even if you are upset.