No killings :3
But it's definitely one of those endings that you can take in different ways. I appreciate all the aspects of Simons personality, and it's one of the reasons why I love the show so much.
At the beginning, when he was young, he was ignored by everyone except Kamina, especially girls. He even had a thing for Youko, but she was interested in Kamina, which was another blow for him. Nia was the only person who he was close to in his whole life, and yes, she was taken away, but to Simon, she probably represented a whole lifetime of love. (Sorry if I'm getting overly sappy here. I fucking love this show and I may read deeper into it than is healthy.)
Simon I'm sure was content with wandering alone. I can appreciate his feelings, as I can identify fairly strongly with his character. You may not though, and that's fine. It's bittersweet, but had she lived, they got married, and had kids...
I'm not so sure I would have liked that. Does that make me a bad person?