Her word was believed by Christopher North, and accepted as proof that Byron had been a great criminal.

Had revenge been her motive, she could have spoken the ONE WORD more that North called for.
The 'Quarterly' asks why she waited till everybody concerned was dead.
There is an obvious answer.

Because, while there was anybody living to whom the testimony would have been utterly destructive, there were the best reasons for withholding it.

When all were gone from earth, and she herself was in constant expectation of passing away, there was a reason, and a proper one, why she should speak.

By nature and principle truthful, she had had the opportunity of silently watching the operation of a permitted lie upon a whole generation.