Speculation is to exactly -who- she killed, and that looks like Hakurei shrine behind her, so my guess is Reimu.
That's not how Hakurei shrine looks, there should be a flat stoneway path leading to it. And no reason for Youmu to do this. I' rather say it's Youmu taking a role of a mercenary-ronin with a mission to exterminate. But yeah, I know, it's just my own imagination.