The motives of Katniss for assassinating Coin was actually to avenge the death of Prim.
<h3>What is assassination?</h3>
Assassination is known to be an act of murdering someone especially a prominent personality or one who is in authority. People are assassinated due to the hatred from others or due to their behaviours that others don't accept.
We can see that Katnissassassinated Coin because of the death of Prim and also because Coin was like another version of Snow who if she comes to power, will make the revolution a failure.