In Game of Thrones Season 6 Episode 10, while torturing Septa Unella, Cersei says these words:
Beating me, starving me, frightening me, humiliating me. You didn't do it because you cared about my atonement. You did it because it felt good.
I do things because they feel good.
I drink because it feels good.
I killed my husband because it felt good to be rid of him.
I fuck my brother because it feels good to feel him inside me.
I lie about fucking my brother because it feels good to keep our son safe from hateful hypocrites.
I killed your High Sparrow and all his little sparrows all his septons, all his septas, all his filthy soldiers, because it felt good to watch them burn.
I understands that Cersei is confessing these things as no one can do anything now to her. She says these to Septa to show how powerful she is now.
Most of the things she said are true but why is she saying she killed her husband, While we know that Robert died during the hunt fighting with a boar? How come Cersei is responsible for that? Indeed she had the most benefit from the death of Robert, but how is she responsible for that?