Addo, of no fixed abode, pleaded not guilty to murder today but guilty to manslaughter through diminished responsibility.
The prosecution said the plea was acceptable because Addo had been severely mentally ill when he killed Mr Marquez. He will be sentenced on Wednesday.
In statement after the hearing, his mother Maria Carmen Marquez Torres, said: "Sergio's life was cut short, a boy that only wanted to work, learn, help others, enjoy the life that God gave him and make everybody that knew him happy.
"He had such a beautiful life. He wanted to be a father and have three children, and the thought of this filled him with excitement.
"Sergio had everything, he was handsome, happy, kind, healthy, responsible, fun, and polite. He is a person that is impossible to replace."
A paranoid schizophrenic killed his flatmate and cut off his head in a frenzied attack after suffering from delusions that he was being persecuted, a court heard today.
Karl Addo, 30, suffered years of mental illness which led to an extreme complex that he was being assaulted and injected with drugs.
He snapped on the day he had been told to leave the flat in Bournemouth, Dorset, he shared with Spanish student Sergio Marquez, 23.
He hit him with a cleaver and stabbed him in what was a "prolonged horrific and very violent attack"
Addo absconded from a mental health unit the day before a tribunal in October 2011 decided he should be sectioned.
He then "disappeared" from mental health services before he moved to Bournemouth in May 2012. He killed Mr Marquez, a hotel waiter, on July 17 last year - nine months later.
The mother of a Spanish student found dead in a flat in Bournemouth last year has paid tribute to her son as a man pleads guilty to his manslaughter.
30-year-old Karl Addo pleaded guilty at Bournemouth Crown Court this morning and is due to be sentenced on Wednesday.
Sergio's mother Maria Carmen Marquez Torres, said:
"As well as mother and son, we were best friends and we looked after each other. When he was young Sergio did not stop repeating ‘I want to be happy’, and he achieved this.
"Sergio’s life was cut short, a boy that only wanted to work, learn, help others, enjoy the life that God gave him and make everybody that new him happy. He had such a beautiful life. He wanted to be a father and have three children, and the thought of this filled him with excitement.
"Sergio had everything, he was handsome, happy, kind, healthy, responsible, fun, and polite. He is a person that is impossible to replace. I would like to thank the people of Bournemouth, and those who have shown their affection with floral tributes and writings.”
A man has pleaded guilty to the manslaughter of Spanish student Sergio Marquez.
The 23-year-old's body was found in a flat in Bournemouth City Centre in July last year.
Today 30-year-old Karl Addo pleaded guilty to manslaughter at Bournemouth Crown Court. He'll be sentenced on Wednesday.
Four men will not face manslaughter charges after a worker died in a 30ft fall at a skip hire and waste disposal company, police have revealed.
Lindsay Campbell, 66, sustained fatal injuries when he fell from the bucket of an excavator at South Coast Skips in Ford on July 25th.
Today Sussex Police said manslaughter by gross negligence charges will not be brought against four men: a 62-year-old from Wickham, Hampshire, a 26-year-old from Fareham, a 47-year-old from Birdham, near Chichester, and a 50-year-old from Bognor Regis.
A police spokesman said that following consultation with the Crown Prosecution Service it was decided there was "insufficient evidence to initiate criminal proceedings for manslaughter".