i dont really understand why katniss kills coin? please explain that to me :) thanks

Prim and some children of the Capitol die in an explosion which makes it look like the Capitol killed them, but when Katniss thinks about the way the children were killed, she remembers the way Gale's traps work. She realizes he could've designed the bomb explosion, killing the innocent children and framing the Capitol as killing their own, therefore losing the trust of their citizens.
Gale denies involvement, but Katniss can't help but hold him responsible. She then thinks it probably was the Capitol, so she confronts President Snow. He tells her that he did not devise the attack because he would've escaped if he did. Then he reminds her that Katniss and him made a promise to never lie to eachother, following the 74th Hunger Games.
At Snow's execution, Katniss thinks over her conversation with Snow and realizes that someone high up would probably have had to give permission for Prim to be on the front lines despite her young age. Making it look like the Capitol killed Prim would push Katniss's loyalty to Coin. This would also drive a wedge between the Capitol citizens and President Snow.
Katniss concludes that it must've been Coin's plan and she had probably aliyopewa the order.
So, instead of killing Snow, Katniss kills Coin for the inhumane act she commited and probably would continue to commit if she lived.

I think that katniss killed coin to say even though she had been fighting snow all the time wasn't her own leader just as bad.