Sarah Sands, 33, was found guilty of the manslaughter of her neighbour Michael Pleasted, 77, after learning he had been charged with sexually assaulting young boys.
A trial in September heard that Pleasted was stabbed eight times and bled to death. Sands was found guilty of his death by reason of loss of control and was sentenced to three-and-a-half years.
Judges at the court of appeal here increased the prison term originally imposed in the case to seven-and-a-half years yesterday, after the case was referred by the attorney general over concerns that the original sentencing had been "unduly lenient", the Guardian reported.
"Following careful review, this case was referred to the court of appeal as it was felt the original sentence was unduly lenient, and the court of appeal increased the sentence to seven and a half years."
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Sands, a single mother of five from east London, killed Pleasted within weeks of hearing that he had been released on bail ahead of his trial on two charges of sexual assault on children aged under 13. He was also under investigation for a further allegation regarding a third boy.
In November 2014, Sands armed herself with a knife and went to Pleasted's flat in Canning Town, east London, to confront him about the allegations, which is when the "determined and sustained attack" took place.
During the trial it emerged that Pleasted, who also went by the name of Robin Moult, had 24 previous convictions for sexual offences between 1970 and 1991, the daily reported.
The presiding judge, Nicholas Cooke, said the case was unique as Sands had lost control rather than taken the law into her own hands and engaged in "vigilante conduct".
Cooke took into consideration Sand's position as a single mother, as well as her admission of guilt when sentencing.
But three judges at the court of appeal ruled that because Sands had taken a knife to the scene, the fact she inflicted fatal injuries and the jury's finding that she must have intended to cause really serious harm to the victim, the sentence should be increased to seven and a half years.