LOL!
So you HEARD she threatened to kill her ex husband? And for this reason you think she might be bipolar?

HILARIOUS!
I think it's possible the ex-husband lied about her threat--lies are not at all unusual during divorces...or she said it in anger-and I'd love to know what happened to evoke that reaction. Couples say really crazy things to one another in anger.
That said...
If you feel uncomfortable leaving your child in her care, by all means DON'T! But please don't let the reason be some malicious rumor. You could easily find yourself on the wrong end of an unsubstantiated rumor one day, wondering why all playdates with your child has ceased.
If you're on the fence, why not have playdates where you guys take the kids out together (ex: Chuck E Cheese) and see what she's like for yourself. You'll also get to see how she interacts with both her child and yours. Or, there's always the option of having her child spend time at your house, which might be a welcomed break for a divorcee.
PS: God forbid you find yourself going through a divorce one day. You'll be surprised at what your husband says about you. As Dr. Phil likes to say, "The person you divorce is not the same person you marry."