Sarah Sands, 32, a mother of five children, killed her neighbour Michael Pleasted, 77, in east London.
Sands armed herself with a knife and stabbed Pleasted in his flat after she learned the old man was charged with sexually assaulting young boys. The man was stabbed eight times and bled to death, Xinhua cited the media reports as saying.
It said Pleasted was on bail for his trial on two charges of sexual assault on children when Sands carried out the attack in his flat in November 2014.
The original case found Sands guilty by the reason of loss of control and gave her a 3.5 year jail sentence.
However, the court of appeal on Friday ruled the sentence was "too lenient" as she took a knife with her and went to Pleasted's flat, which means the starting point for sentencing should have been 10 years.
After Sands handed herself over to the police, she told a police officer Pleasted had touched some children "so I took care of it -- I stabbed him."
She said she had not intended to hurt Pleasted but he "smirked" when he answered the door and told her the boys were all liars who had ruined his life.
"Our office received a great deal of correspondence in regards to this highly emotive and sensitive case," said a spokesman for the attorney general's office.
Following careful review, the case was referred to the court of appeal where the sentence was raised to 7.5 years, he added.
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