Aras Hussein, 21, had allegedly blackmailed 18-year-old Reema Ramzan by taking sexual photographs of her and threatening to share them with her family if she broke up with him.
Sheffield Crown Court was told that on June 4 last year, he killed her by cutting off her head with a knife before inflicting stab wounds on himself.
Prosecutor Graham Reeds QC outlined the case against Iraqi-born Hussein, who denies murder and actual bodily harm.
‘Bruising into tissue wounds suggest that Reema was still alive as the defendant started to remove her head,’ the lawyer said.
‘The process of decapitation would require considerable motion with corresponding considerable force which needed to be sustained until all the tissues had been cut through.
‘Force would also be needed to restrain the victim who is likely to have been struggling. Severe force would have been needed to cut through the spinal vertebrae with a knife in order to remove the head.’
Mr Reeds told the jury that Hussein was seen naked outside his flat in Sheffield with blood pouring out of his chest, and told emergency workers: ‘I don’t know why I did it.
‘She like me but I raped her. What I did was wrong. I need punishing for it.’
He also allegedly asked the paramedics: ‘Why are you helping me? I’ve murdered someone.’
The lawyer said Hussein was likely to claim he was suffering from diminished responsibility due to schizophrenia at the time of the attack.