A woman who was murdered by her ex-partner had described her killer as "the best guy ever", her brother has said.
Cristina Magda-Calancea, 26, died after being stabbed multiple times in King's Lynn, Norfolk, on 21 September.
At Norwich Crown Court, Gediminas Jasinskas, 30, admitted murdering Ms Magda-Calancea, who he had previously been in a relationship with.
Her brother, Razvan Milea, said Ms Magda-Calancea's family wanted answers.
Jasinskas has so far refused to reveal the motive behind the killing. His relationship with Ms Magda-Calancea had ended a month before she was stabbed.
Mr Milea - who worked at the same factory as Ms Magda-Calancea and Jasinskas - said he "can't believe what happened" and that the killer "doesn't want to say why" he did it.
Mr Milea said: "He was a perfect guy - polite, always on time, didn't smoke, didn't drink, didn't do drugs.
"We spent time together at work and at home. Everything looked perfect - even my sister said that.
"She called him the best guy ever and that he understands her.
"Everything was normal, but then some things happened that we didn't know about [until after her death]."
Mr Milea said the couple had argued and fought, and after one particular altercation he had told Jasinskas "if you break her heart, I'll break your neck".
"He said 'no, I love your sister, I'm a good guy, I respect her and your family' - but look what happened."
Ms Magda-Calancea and Jasinskas had been in a relationship for almost a year but her brother said it ended after his sister "couldn't handle" his "jealousy".
Mr Milea said: "He was jealous of everyone - men or women, old or young. Even when my twin brother went to chat with her."
He said that five days before her murder she had told a friend Jasinskas "doesn't leave her alone and all the time is harassing her and texting and calling her".
Jasinskas, of Tennyson Avenue in King's Lynn, will be sentenced on 3 January.