I wouldn't think of Diana as evil, just extremely vengeful. That's definitely not a GOOD trait to have at all, but it doesn't make her evil.
The tradition of the Rakkor is by far more violent, Leona refused to kill the dude she beat so she had to be executed in his stead, and she fully accepted that.
Diana just came in, and was all happy that she was right all along and then the Solari turned on her... why would they execute her for showing them the truth?
Diana isn't evil. If anything, the Rakkor are violent and the Solari are corrupt.
I can't wait until this Solari/Rakkor storyline is touched upon and Diana and Leona eventually confront each other; you just know Diana will go bonkers trying to kill Leona, but I imagine Leona wouldn't want to kill Diana. What'll happen if the Solari higher-ups find out their symbolic warrior doesn't want to get rid of their "Lunar Executor"?