Double killer Derek Boyd has been jailed for life for murdering a man at his north Dublin home and has been told his sentence for the manslaughter of his sister four years ago will now be referred back to the Circuit Court.Boyd (31), with an address at Scribblestown Place, Finglas, Dublin 11, was serving the suspended part of his sentence for killing his sister when he fatally stabbed father-of-four Mark Carroll (34) in the chest. On Tuesday, he was described by the victim’s family as a “monster”.In an emotional victim-impact statement delivered to the Central Criminal Court, the deceased’s family said Carroll was taken from them in “a vicious and unnecessary act of violence”.The deceased’s father, Kevin Carroll, told the court the family had to go through a “painful and soul-wrenching trial”, where they had to listen to evidence of Mark Carroll’s “sickening injuries”.A Central Criminal Court jury took nearly nine hours last week to find Boyd guilty of murder, unanimously accepting the State’s position that self-defence was not open to the accused.The prosecution had argued that Boyd chose to pick up a knife in “a determined, murderous attack” on June 9th, 2024.In his closing address, Sean Guerin SC, prosecuting, said Carroll’s return to Boyd’s home moments after he initially left “wasn’t remotely threatening but entirely reasonable”, as he was going back to “sort out” an earlier row.Boyd fled the jurisdiction by boarding a ferry from Belfast to Scotland just hours after he killed Carroll. He had also fled the scene of his sister’s killing in March 2022, later telling gardaí he felt ashamed for leaving her like that.At his sentencing for killing his sister Sandra Boyd, the Circuit Court heard Boyd was assessed as being “at low risk of homicidal reoffending”.In a statement given to gardaí more than a year after Carroll’s death, Boyd said he believed his life was at risk and that he acted in self-defence when he carried out the fatal stabbing. The defence said Carroll had let himself back into Boyd’s home after being ordered to leave just minutes beforehand. Boyd also said “given what had happened with his sister”, he “did not want to harm anyone intentionally and certainly did not intend to end someone’s life”. He took the stand at the trial, telling the jury how he picked up a knife and “begged” Carroll – who he’d been drinking with that day – to leave his home, claiming the deceased repeatedly threatened to kill him.“He told me ‘I’ll stick the knife up your hole’,” Boyd said of the deceased. Boyd said Carroll grabbed him and, during a struggle, tried to take the knife. The defendant said he managed to “break free” and swung the knife at least three times but never intended to kill or seriously injure Carroll.“Exactly what passed between them in the house was not established on the evidence,” Guerin noted on Tuesday.A pathologist had told the trial Carroll died from a stab wound to the chest, which measured around 10.5cm in depth and was unsurvivable. The expert witness also said Carroll had minor injuries consistent with being in a fight. Handing down the mandatory term of life imprisonment for murder on Tuesday, Ms Justice Melanie Greally said Boyd’s sentence was to date from August 5th 2025, when the defendant went into custody.In relation to the suspended sentence Boyd was serving for killing his sister when he murdered Carroll, the judge said she would list the case for re-entry before the Circuit Court on Thursday. Boyd had pleaded not guilty to murdering Carroll. In seeking a verdict of manslaughter or an acquittal for his client, Brendan Grehan SC, with Seoirse Fennessy BL, defending, had argued that Boyd “acted out of instinct” when he fatally stabbed Carroll. Counsel argued Carroll had “trespassed” on Boyd’shome and that the defendant was entitled to protect himself if he was under attack.Carroll had come to Garda attention over the years and had pleaded guilty to assault causing harm to his girlfriend in her home, where gardaí noted on arrival that she was bruised, bloody-faced and visibly shaking.At the time of his death, Carroll was also awaiting trial on a second, unrelated charge of assault causing harm.Det Sgt Niall Murray, of Finglas Garda station, told Guerin that Carroll was from Finglas but had lived in Ashbourne in Co Meath, as well as in Co Monaghan with his father.Guerin said Boyd and Carroll were known to each other through a family connection, but “not very closely, it seems”.The prosecutor said the murder weapon was never recovered and that Boyd told gardaí he had taken it with him from the house. Under cross-examination by Grehan, the officer agreed that Carroll had only left the property at Scribblestown Place four minutes earlier – having previously been ordered out by Boyd – when he had returned.Grehan said it appeared from how the two men were getting on that day, as well as their previous knowledge of each other, that the murder was “motiveless”. “Yes, senseless,” replied Det Sgt Murray.Greally said this was undoubtedly a “senseless and motiveless” killing. The judge described it as “an act of anger” which has had “the most catastrophic consequences” for all those connected to Carroll.