"Being such a lady could I weep these tears If this were true? A great queen such as I Having sinn'd this way, straight her conscience sears;
"And afterwards she liveth hatefully, Slaying and poisoning, certes never weeps: Gauwaine be friends now, speak me lovingly.
Instead of going back to her story, Guenevere continues to address Sir Gauwaine.
She says that, because she is a "lady" and a "great queen," of course she'd feel terrible after committing a "sin" like kissing Launcelot.
After the fact, she could only live "hatefully."
She says she was "slaying and poisoning," but we have to assume she doesn't mean that literally. After all, she's not on trial for murder, she's on trial for adultery, for cheating on her husband.
Perhaps she means that her life felt poisoned because her conscience was bothering her.
She asks Gauwaine to be friendly and "loving" to her.