In the Mishna, Sotah 1:3 and 3:3 both speak of instances in which a woman suspected of having committed adultery admits of her crime before drinking the water. Given that the penalty for adultery is death (eg: Leviticus 20:10, Sanhedrin 11:1, etc), and given that the curse of the bitter waters might not take effect immediately (cf: Sotah 3:4), under what circumstances would a guilty woman confess to her crime? Since there are no witnesses, would not every woman choose to take her chances with the water?
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if she confesses she won't be put to death by Beis Din since you need 2 witnesses for that.
and if there are 2 witnesses then she won't be tested by the sota water, hence there are not 2 witnesses.
therefore, she would be saving her life from the water by confessing and would not be executed by beis din.