Nicola Moss told how Alan Burbidge, 69, was one of the most "able-bodied" patients in the hospital - but who died after being given drugs by fellow nurse Phillip Reed.
Reed, 43, is accused of attempting to murder Alan Burbidge while his wife Joyce was at his bedside.
The court heard Reed tried to kill Mr Burbidge with a dose of the morphine-based drug Oramorph twelve times stronger than he was prescribed by a doctor.
Reed is on trial for two attempted murders because he allegedly wanted power over life and death.
Mrs Moss told how she cared for Mr Burbidge while she was training at the hospital and described him as one of the most "able-bodied" patients.
'very upset'
She was "very upset" when she was told by a fellow trainee, Sarah Thomas, of Mr Burbidge's death.
Mrs Moss told the court: "Sarah Thomas told me Mr Burbidge had died and I was very upset as I had cared for him a lot.
"He was my patient and in the beginning Mr Burbidge was mobile and could feed himself and I used to talk to him a lot.
"He was one of the most able-bodied patients there and he suddenly deteriorated quite quickly."
Ward Manager Amanda Jenkins described Reed as a "good" nurse and claimed he told her he did not try to kill Mr Burbidge.
Mrs Jenkins said: "From what I saw he had a caring attitude.
"I have never seen anything untoward in his behaviour.
"He rang me at home and he said Sarah Thomas had made an allegation of overdosing Mr Burbidge.
"I said: 'Did you do it?' and he said: 'No'."
Reed is accused of trying to murder Mr Burbidge, at Tonteg Hospital near Pontypridd, south Wales, where he was a senior nurse, and another elderly man at a nursing home in which he worked.
Funeral arrangements
Mr Burbidge's widow Joyce Burbidge, told how the pair had run a post office for 20 years.
And she said she would regret to her dying day not challenging Reed when he told her he would give her husband something to: "help him on his way".
The court heard Mr Burbidge's relatives left the room while staff tried to cool him down, and he died shortly after, with his wife by his bedside.
The court heard that four weeks after Mr Reed became a full-time patient at the hospital, Reed asked Mrs Burbidge about funeral arrangements.
Mr Burbidge's step-daughter, Lianne Burbidge-Edmunds, claimed Reed attended the funeral but could look her in the eye.
Prosecutor Nicholas Cooke QC said Reed tried to kill the patients while working in both the hospital and a nursing home.
Ill-treatment accusations
Mr Cooke said: "He had a morbid interest in death. Working close to death, he wanted to have power over when it would occur."
Reed is also accused of trying kill David Llewellyn Parker, 69, at the Bedwellty Nursing Home in Aberbargoed in the Rhymney Valley.
Reed is also accused of ill-treating another two patients. He allegedly gave nursing home resident Ken Edwards three injections of non-prescribed drug.
Another resident Olwen Thomas, who suffered from dementia, was with-held drugs prescribed to her after she became aggressive, the court heard.
Married Reed, of Caerphilly, denies two charges of attempted murder, two of administering poison as to endanger life and four charges of ill-treating a patient.
The case continues.