A Dix Hills man was defending himself when he killed his brother and hurt his mother during incidents that were “accidental, unfortunately reckless,” but not intentional, his attorney argued Thursday.

But a Suffolk prosecutor said Charles Okonkwo Jr., 21, meant to strangle his 15-year-old brother and seriously injure his mother in their home in 2014, and Okonkwo didn’t fear either of them.

“The blame . . . for the ruin of the Okonkwo family sits right there at that table,” Assistant District Attorney Glenn Kurtzrock said in closing arguments during Okonkwo’s weeklong trial in a Riverhead courtroom.

Kurtzrock added: “This was no accident, folks. This was no mistake. This was no reckless act. This was murder — plain and simple.”

Okonkwo is charged with second-degree murder and first-degree assault. However, State Supreme Court Justice Mark Cohen will allow the jury to consider a lesser charge of second-degree manslaughter if they find him not guilty of murder.

Prosecutors said the defendant’s father, Charles Okonkwo Sr., returned home on July 19, 2014, after a brief hospital stay. He discovered his son Bradley dead in his bedroom and wife, Chinwe, 54, unconscious in a pool of blood, with cuts and facial fractures, in the garage.

Authorities said Okonkwo Jr. admitted to hurting his brother and mother.

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Prosecutors said Okonkwo Jr. ran from the home and was found at a nearby shopping mall.

Okonkwo’s father, along with his mother, who was in a wheelchair, and his pastor sat in the courtroom Thursday.

Defense attorney Eric Besso of Sayville said his client had lost his wallet, and his younger brother didn’t want to be asked about it and “be accused, so they had a fight.” During that fight in Bradley’s bedroom the younger brother “choked” Okonkwo.

“Charles tried to pull his arms off of his neck, not being able to do so, he reached behind him and pressed on Bradley’s neck until Bradley let go of Charles’ neck,” Besso said.

Scared, Okonkwo ran to his mother for help but she “immediately turned her ire and rage on Charles . . . She swung a lamp at him,” Besso said.

She chased her son around the house, and he grabbed a steering wheel lock that she was hitting him with in the garage. In a “knee-jerk reaction,” he hit her, Besso said.

“He reasonably believed he was going to get hurt by his brother and he reasonably believed he was going to stop his mother from hurting him as well,” Besso said.

“It’s a homicide. There’s a death, yes, but it’s not murder,” Besso said. “There’s an assault, yes, but it’s not an intentional assault.”

But Kurtzrock said Okonkwo testified that he didn’t fear for his life with his mother or brother.

“What the defendant did was he grabbed his brother by the neck and he strangled him until he was dead,” Kurtzrock said.

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Kurtzrock said the county’s chief medical examiner testified that it would take several minutes at least to strangle someone. “That’s a long time to have your hands on someone’s neck, and squeezing it and squeezing it,” Kurtzrock said.

Kurtzrock said Okonkwo also had already taken the steering wheel club from his mother when he attacked her, “so there’s no longer any need to defend himself.”

“The injuries to his mother . . . they’re so severe, they are so excessive and brutal that there is absolutely no evidence, even from the defendant’s story, that he needed to go to that extent to protect himself,” Kurtzrock said.

The jury went home after deliberating for about two hours. Deliberation continues Friday.