The sex testimony will be what convicts her in my opinion.
It's when she comes closest to revealing her true nature. There's little defiance, anger, embarrassment or shame. Sex feeds her - it's her source of supply. Reliving these accounts she also relives the power she can hold over others. Superiority, narcissism, power, control, greed, manipulation.
We see it all - every single bit of her nasty, evil self - when she's recounting their sexual exploits. I love that Juan is using her biggest fantasy as the weapon that will seal her fate.
JMO