“I have noticed that my emotions, the way I behave, the way I react, is not the same as others, it is always aggravated,” Francois Patrick Nogueira Gouveia, 21, said as he took the stand at a court in Guadalajara, about 60 kilometres (40 miles) east of Madrid.
“If I could control it, it would be wonderful, not only for me but for others. I would have liked it if none of this happened… I did not chose to function the way I function,” he added.
Gouveia, then aged 19, fled to his native Brazil shortly after killing his uncle and aunt, and their two young boys, aged just one and three, at their home in the village of Pioz near Guadalajara in August 2016.
He returned to Spain in October 2016 and turned himself in to police after the corpses of the family were found in plastic bags at their home in Pioz.
Gouveia's uncle and aunt, both from Paraiba state in Brazil's northeast, had been dismembered.
He exchanged messages and pictures of the corpses with a friend via WhatsApp while he cut the bodies of the two adults into pieces before putting them in the plastic bags.
Police said he told them at the time of his arrest that he had an”uncontrollable desire to kill”.
Gouveia lived with the family for four months in Torrejon de Ardoz, also near Madrid, before the couple moved to a quiet residential area in Pioz.
Marcos Campos Nogueira and his wife Janaina Santos Americo were killed alongside their two young sons. Photo: Facebook
He told the court his uncle angered him because he told his family back in Brazil that he had had alcohol and drugs in Spain.
“That was it for me and I said to myself 'this is not going to stay like this',” Gouvea told the court.
Prosecutors have asked that he be handed a life sentence, which in Spain means he would serve a minimum of 25 years behind bards.
The defence argues Gouveia suffered a “temporary mental disorder” and hopes his confession will mitigate the sentence.
The court is expected to hand down its verdict and sentence in about a week.
By AFP's Diego Urdaneta