A father reportedly threw his 11 months old son into River Irwell in Radcliffe, Greater Manchester.
Zak Bennett-Eko, 23, threw baby Zakari Bennett-Eko into the river before heading to a lock keeper pub where he was arrested.
The baby died after he was taken out from the river by an emergency crew at 5:15 pm in September last year.
A six-day trial held at the Nightingale court sitting at the Lowry theatre in Salford heard Bennett-Eko, who suffered from paranoid schizophrenia, relate his child was turning into the devil.
Bennett-Eko defence claims because of his sickness, he did not know what he was doing wrong and should not be found guilty because of his insanity.
Bennett-Eko, who was diagnosed with psychosis related to cannabis at the age of 17, missed GP appointments following Zakari’s birth.
In text messages heard in court, his girlfriend Emma Blood told friends he had kicked her door in, thrown things at her and claimed pop star Beyonce was his mother.
In one message, she said: ‘He’s saying Zakari isn’t our baby and stuff. One second he’s OK, the next minute he’s going mad saying he’s going to kill himself.’
Miss Blood who was pregnant with a second child, had told Bennett that he needs to take care of himself, that she could not take care of him and two babies.
Rob Hall, the prosecutor opening the case, said ‘It may be that exchange set the seed in Zak’s mind that they would be better off without Zakari.’
Meanwhile, psychiatrist Dr Inti Qurashi told the court that the defendant told him he hesitated before throwing Zakari into the river and thought: ‘What the f*** am I doing.’
Dr Qurashi added he considered the hesitation before the baby was thrown into the river an important point and said it showed the defendant knew what he was about to do was wrong.
Dr John Crosby, Bennett clinician, told the court the defendant believed he was throwing the devil into the river.
The jury retired just after 12pm on Monday and returned its verdict of guilty of manslaughter shortly after 3pm.
The case has been adjourned to Tuesday when the defendant will be sentenced.