Neil Sedaka "Bad Blood" lyric question

Bad (bad) blood (blood)
The bitch is in her smile
The lie is on her lips
Such an evil child

What does “the bitch is in her smile” mean?

I always took that to mean that while she is smiling as if she was happy to see you and be with you, that it was merely a manipulative ploy on her part, and that she wants something from you.

That makes pretty good sense. Thank you.