R.B.
If it's only been a few days, I don't think you should make too much of it. However, I would beware of labeling her and creating a perfectionist complex. If you talk a lot about her being "good" and "smart" and "honest" and things like this, it sets a really high standard that she feels she has to continually live up to. It's not healthy to set the bar so high that it feels unreachable.
Start praising her for effort, and stop labeling her with innate qualities. Read up on Carol Dweck and Growth Mindset.
I agree with Diane that you shouldn't dismiss her thoughts by telling her to think of happy things, instead you could casually say something like, "Hmm, that's interesting, well everyone sometimes has some strange thoughts, I wouldn't worry too much about it." Treating things lightly usually has the effect of making the child treat it lightly and being able to move on.
If this keeps up and she keeps talking about unwanted thoughts for a few weeks or months, you could have her evaluated. But it's too soon to make much of this.