She has written a heart-wrenching letter to Attorney-General Chris Finlayson asking him to ensure Ewen Macdonald is given a sentence that reflects the gravity of his offending when he appears in court this Friday.
"I need a sentence that reflects appropriate justice," Mrs Guy said in her letter, which was copied to Prime Minister John Key and Justice Minister Judith Collins.
Macdonald earlier pleaded guilty to six charges relating to damaging and destroying property - two relating to Mrs Guy and her late husband Scott.
Mrs Guy wants Macdonald to serve time for each crime, rather than serving all of his sentences simultaneously.
Scott Guy was gunned down in the driveway of his Feilding home in July 2010. Macdonald, who is married to Mr Guy's sister, was charged with murder but acquitted by a High Court jury.
Macdonald admitted arson and causing damage to Mr and Mrs Guy's almost-completed new home in a bid to scare them. He smashed holes in walls, broke all of the windows and left obscene messages scrawled on walls.
Mrs Guy has said she wants to stand up for "Scotty" and Macdonald's other victims and make sure the suffering and distress he caused them is acknowledged and reflected in the sentence he is given. Protecting her young sons Drover, who she was pregnant with when her husband died, and Hunter is also important.
She told the Attorney-General: "If the offender is granted a concurrent sentence it will not reflect the suffering he has caused to my family, neighbours and the community and will mean that he is able to escape the true gravity of his actions."
A cumulative sentence would mean Macdonald served his individual sentences end-on-end. A concurrent sentence would see him serving time for each offence simultaneously.
"He needs to pay for all of his individual and separate crimes and the hurt, pain and suffering he has caused - not only to myself, my husband and our children, but to numerous other people within New Zealand," said Mrs Guy's letter.
Sensible Sentencing Trust spokeswoman Ruth Money said she supported Mrs Guy's call:
"If I drove from work to home and got three speeding fines on the way, I would have to pay three fines - not one."
She feared a concurrent sentence would reinforce to other offenders that they got "free hits" after the first event.
Macdonald has been in custody since his arrest in April last year. The longest sentence he could be given is 14 years - the maximum penalty for arson. He may be given a reduction for his early guilty plea and time served in prison.
His accomplice, Callum Boe, a former worker on the Guy family farm, was sentenced to 19 months after pleading guilty to the same charges.
A spokesman for the Attorney-General said he had not seen the letter and it would be improper to offer any comment on a case before the courts.
