This is a VERY foolish statement. How do you KNOW she did it, but you can't find any proof? Did you see a video of it? Did she confess to it? Is there substantial evidence that leads one to believe she did it? Any of these things would also be evidence they could have used in the trial, but none of it existed. You have no proof, so can you explain how you KNOW without a doubt that she killed her daughter?
This is the sole reason why lawyers will spend hours talking with the potential jurors, making sure they find someone who is COMPLETELY unbiased. Not somebody who says "Oh she must have done it, she didn't report her daughter missing right away" because that doesn't prove anything. In a trial, you need to look at actual evidence, and leave your opinion of the person out of it.