The jury in the trial of Michael Scott can return a verdict of manslaughter if they acquit him of murder but find that he was grossly negligent when he reversed over his 76-year-old aunt in an agricultural teleporter.
Ms Justice Caroline Biggs has begun her charge to the 15-person jury in which she explained the legal principles that they will apply when considering the evidence.
To find Mr Scott guilty of murder, she said they must be satisfied beyond reasonable doubt that Mr Scott intended to kill or cause serious injury to his aunt when he ran over her.
If they are not satisfied that the prosecution has proven the case for murder, Ms Justice Biggs said the jury must consider a verdict of manslaughter through "gross negligence".
If there is a reasonable possibility that what happened was an accident, they must acquit Mr Scott and enter the words "not guilty" on the issue paper, she said.
For a manslaughter verdict the prosecution does not have to prove that Mr Scott intended or even foresaw that he was going to harm Ms Treacy or anyone else, the judge said. "It is the act itself of driving in a grossly negligent way causing the death of another human being that gives rise to manslaughter," she said.
A finding of criminal negligence would require the jury to be satisfied that the manner of Mr Scott's driving was "so bad that any reasonable person, if they thought about it at all, would have realised that they could cause serious injury to some person."
Mr Scott (58) of Gortanumera, Portumna, Co Galway has pleaded not guilty to murdering Ms Treacy outside her home in Derryhiney, Portumna, Co Galway on April 27th, 2018.
The trial has heard that Mr Scott told gardaí that he was reversing the teleporter across the yard outside Ms Treacy's home when he felt a "thump" and thought he might have struck a trailer.
He said he rolled the machine forward to level ground and when he got out of the cabin he found Ms Treacy lying on the ground.
The prosecution case is that Mr Scott deliberately reversed over Ms Treacy following a long-running dispute over land. Mr Scott's lawyers have told the Central Criminal Court that her death was a tragic accident.
Ms Justice Biggs on Wednesday told the jury that they have a "tremendous burden" but must not shirk from their responsibility in coming to a verdict.
They must not be concerned about the consequences for Mr Scott if he is convicted of an offence, or allow sympathy for Ms Treacy to influence them if their decision is to acquit.
"If you are doing that you are not applying the cold clinical assessment of the evidence," she said. "It is important to separate your decision-making process from the consequences of your decision."
If they find that there is a reasonable possibility that Ms Treacy's death was an accident, the prosecution has not proven its case to the standard of beyond a reasonable doubt and the jury must acquit.
If on any part of the evidence there are two views available the jury must accept the one favourable to the accused unless the prosecution has proved its version.
Even if the defence version is less likely, she said, if it is reasonably possible the jury must give the benefit of that doubt to the accused.
Ms Justice Biggs said the prosecution had pointed to alleged lies told by Mr Scott as evidence of his guilt.
Dean Kelly SC, for the prosecution, said that Mr Scott lied when he said his relationship with Ms Treacy was good, that Ms Treacy was still breathing after being run over and that he didn't know he could get help for her by dialling 999.
Evidence of guilt
Ms Justice Biggs said that if they find that Mr Scott lied, the mere fact that he lied is not evidence of guilt.
People lie for different reasons including embarrassment, confusion, to protect others, or to conceal matters that look bad but are not related to the alleged offence, she said. If there could be an innocent explanation for a lie, she told the jury to ignore the lie.
For the jury to rely on a lie as evidence of guilt, they must first be satisfied that the lie was deliberate and was not told for any innocent purpose, "but because he knew the truth would implicate him," she said.
She added: "If you are satisfied that he lied deliberately and if you are satisfied to the standard of beyond a reasonable doubt that the only reason for the lie was to cover up his guilt, you then can rely on those lies in support of the prosecution case."
Ms Justice Biggs said the jury had heard evidence of acts allegedly done by Mr Scott including that he failed to take away Ms Treacy's rubbish, causing it to build up in her yard, and that he switched off the oil to her heating system during a "big freeze" in spring 2018.
If the jury is satisfied that Mr Scott was responsible for such acts, the judge said that would be defined as "misconduct evidence". Its purpose, she said, is not to suggest that Mr Scott is of bad character and therefore more likely to have killed or murdered Ms Treacy.
Its purpose, she said, was to give the jury a complete picture of the relationship between the accused and deceased prior to the alleged offence.
Turning off her oil or allowing rubbish to build up in her yard would be a "mean, nasty thing to do," Ms Justice Biggs said. If the jury is satisfied that Mr Scott did those things, "that does not mean that he is someone who therefore has the propensity to kill."
The prosecution contends that the background evidence rebuts alleged lies told by Mr Scott to gardai that he had a "great relationship" with Ms Treacy and that it rebuts his defence that what happened was an accident, the judge said.
The prosecution case is a circumstantial one, the judge said, but that does not suggest that it is based on substandard evidence. Circumstantial cases require the jury to ask themselves whether the cumulative effect of all the acceptable evidence proves the accused's guilt beyond all reasonable doubt, to the exclusion of all other rational explanations consistent with innocence, the judge said.
Ms Justice Biggs will spend two to three days recapping the evidence heard during the trial which began in January.
When she has completed her recap, three jurors will be chosen by lottery and discharged, leaving 12 to consider their verdict.