I'm afraid it looks as if she might have been about as admirable as Lady Macbeth, if all the rumours are true!
From the Chronicle of Florence of Worcester:
[a.d. 793.] Ethelbert, the most glorious and holy king of the East-Angles, whose eminent virtues rendered him acceptable to Christ, the true King, and who was courteous and affable to all men, lost at once both his kingdom and his life, being beheaded by the detestable commands of Offa, the mighty king of Mercia, at the infamous suggestion of his own wife, queen Cynefrith; but though iniquitously slain and deprived of his kingdom, the king and martyr entered the courts of the blessed spirits, while the angels rejoiced in triumph. The consecration of archbishop Ethelhard was celebrated on twelfth of the calends of August [21st July].
The full grisly story as recounted by Roger of Wendover!:
About the same time, Ethelbert, king of the East Angles, son of king Ethelred, quitted his own kingdom, in spite of the strong remonstrances of his mother, and came to Offa, the most mighty king of Mercia, and begged of him that he would give him one of his daughters in marriage. And Offa, that most noble and most illustrious and most high-born king, when he had learnt the object of the arrival of king Ethelbert, received him with great honour in the royal palace, and showed him all the attention and kindness that lay in his power, not only to the king himself, but to all his comrades who had come with him. But when king Offa consulted his queen Quendritha, and asked her advice on the subject, she, being urged by the promptings of the devil, is said to have made answer to her husband, " Behold, God has this day given your enemy into your hands, whose kingdom you have so long coveted with daily desire, so that now you can extinguish him secretly, and so his kingdom will pass under the power of you and your successors for ever." But the king was greatly agitated at the advice of his wife, and reproving her with indignation, made answer to her, " You have spoken like one
of the foolish women; far from me, far from me may so detestable an action be, which, if it were perpetrated, would be an everlasting reproach to me and my successors." And, having said this, the king departed from her. Afterwards, when his agitation had become gradually calmed, both the kings sat down at table, where, having refreshed themselves with royal food, they spent the whole day with music, and dancing, and harp-playing, to their great delight. But in the meantime, the wicked queen, not abandoning her foul design, treacherously ordered a bed-chamber to be adorned in royal fashion with silk mattrasses and curtains, for king Ethelbert to pass the night upon ; and near the royal bed she caused a chair to be made ready, furnished with the most princely decorations, and surrounded on all sides by curtains, beneath which, wretch that she was, she caused a deep hole to be dug, in order to effect her wicked purpose. Accordingly, when king Ethelbert, after a day of pleasure, wished to give up his limbs to sleep, he was conducted into this bed-chamber, and, as soon as he sat down in the chair which I have described, he was suddenly precipitated into the deep hole, chair and all, and strangled by the executioners whom the queen had concealed there. In the moment that the king had fallen into the pit, the wicked traitors threw over him pillows, and garments, and curtains, that his cries might not be heard. And thus that king and martyr, being murdered, though innocent, received the crown of life which God has promised to those that love him.
But when this detestable action which the wicked queen had done to the suitor of her daughter became known to the comrades of the murdered king, they departed from the palace before daylight, fearing lest they themselves might be subjected to similar treatment. And the noble king Offa, when he had received information of the crime that had been committed, mourned, and shut himself up in a chamber, and for three days would not taste food. But, although he was quite innocent of all participation in the king's death, he nevertheless sent a powerful expedition, and annexed the kingdom of the East Angles to his own dominions. And the holy Ethelbert was buried without any honour, and the place was known to no one, till his body, having been pointed out by light from heaven, was found by the faithful, and was conveyed to the city of Hereford, where it now adorns with its miracles, and glorifies with its virtues, that episcopal see.