assassinated

To secretly kill someone, usually an important person.  The killing is usually because the killer and person killed disagree about politics.


make gentle the life of this world

This is a poetic way to say that people and things will be gentle and kind to each other and to the world.  Poems and the words in them can have more than one meaning, so it could also mean something different to you.


polarization

This word means that things become separated into different groups rather than being together.  During this speech, Robert was worried that the death of Dr. King would make black people and white people divide into two groups, or polarize, rather than live peacefully together.


revenge

Revenge means to hurt someone who hurt you.  It is different than wanting someone to get justice, which is like when a person wants a theif to go to jail.  Revenge would be the person wanting to hurt the theif or to steal from the theif so he would know how it feels to have something stolen from him.  Revenge usually causes anger and violence, which is why it is not good to do.


riot

A riot is when a group of people become angry about something and show their anger by going into the streets and yelling and breaking things.  Sometimes rioters, or people who are in a riot, even hurt other people.


A riot in Spain
http://www.ibtimes.com/spanish-protests-devolve-riots-795751

savageness

Savageness is acting in a violent and animal-like way.  It usually means not caring about hurting other people.


stain of bloodshed

This is a poetic term to refer to the violence in the United States because of racial and civil rights disagreements.