Maureen Kennedy blamed the drug for her nephew Garry Kane's killing of her mother Kathleen Milward, 87, after he was found guilty of the murder at the High Court in Dumbarton.
She said the family had gone through "unspeakable and unbearable pain" since she was found dead at her bungalow in Stonehouse, South Lanarkshire, in January.
But Mrs Kennedy admitted she did not completely blame Kane, and believes his victim would even have forgiven him.
She said: "I don't hate my nephew, but I hate what he has done.
"I know that his gran would be shocked and appalled if she was here at a terrible crime like this, but I also know she would forgive him.
"He has caused unspeakable and unbearable pain to all his family, and although I'm glad that justice has been done, there are no winners in this tragic situation."
Kane faces a life sentence for the killing, but his aunt said: "I believe that, ultimately, drugs, mainly heroin, killed my mother.
"Drugs are a curse to society, taking lives, ruining lives and destroying relationships."
"Highly and quickly addictive heroin and the like starts off as a pleasurable experience, but it is a poisoned chalice. It leaves addicts leading a desperate and miserable life, with addiction that knows no boundaries as to how far they are in order to go to get their next fix.
"Garry Kane is an addict and he is proof of this. His life became a daily struggle to get from one fix to the next."
She said that after his mother had thrown him out, Kane turned to Mrs Milward for money.
However, "when she eventually told him that she would not give him any more money she paid the ultimate price".
Kane hit her at least three times with a blunt object and rained blows on her frail body.
She lay dying in a pool of blood with 26 injuries including three blows to the skull, horrific bruising, a broken cartilege in her neck and a broken finger.
Meanwhile, Kane left to meet his dealers.
Mrs Milward's error had been to take pity on her relative. She gave him a roof over his head after his own mother had thrown him out for stealing.
Kane, 41, returned to the house after the murder and made a distraught phone call to his mother and the ambulance service, sobbing and gasping down the phone as Mrs Milward, 87, lay on her kitchen floor.
When asked in court if he had killed her, Kane said: "I couldn't do something like that to my granny."
His mother Kathleen Kane, 64, cried openly as she told jurors that "Garry's granny" meant the world to him, and he would never have harmed her.
But the jury found him guilty of murder in a majority verdict.
Gasps from the public benches greeted the verdict after the three-week trial.
Kane scowled and shook his head, while Mrs Kane shouted "no" and sobbed inconsolably.
Deferring sentence for background reports, judge Lord Matthews said: "As far as the sentence is concerned, the only one I can impose is life imprisonment. There is a certain amount of time that must pass before you can have parole."
Expert evidence analysing the contents of Mrs Milward's stomach showed that she died while Kane was there, despite his denials.
Kane's DNA was found under Mrs Milward's fingernails after one of them was broken off as she tried to fight back.
When asked if he was responsible, Kane said: "I'm sorry. I don't know who did that, I just know I didn't do it."