I've read the Mockingbird book twice, and I haven't found a solid proof yet, that Coin is a bad person. Only the opinion that Katniss thinks she is (but that might be a manipulation of Snow's). The real streangth of Katniss is that she is also flawed.
When all comes down to it, I'm still not convinced that Coin killed Prim on perpose. She might have killed her accidently, but that makes her as guilty as both Gaile and Beetee (Beetee is my favorite Charachter!!!).
I tend to believe that Prim was at the front-line because she wanted to be there (her older sister setting an example of fighting for what you belive in!). And all in all, the facts that have to be considered to be able to to predict the possibilty of Prim being at the frontline when the second bombs explode, is very slim!!! Which makes me conclude that Coin wasn't able to predict that Prim would be at the front-line when the bombs exploded.
Overall, I don't think that district 13, nor Coin was evil!!! I do, however think that the killing of Coin was an overall good thing, but that is a howle other matter