A DAD diagnosed with a terminal brain tumour killed his son because he feared his wife would not be able to raise him alone, cops say.

Oleg Drachev, 40, allegedly knifed 13-year-old Ilya in his sleep after his wife had gone to work in Moscow.

8 Oleg Drachev is accused of murdering his son Ilya, 13 Credit: VK.com

8 He also allegedly tried to stab his wife but she fought him off Credit: VK.com

Later she rushed home worried when he did not answer the phone, broke in through a window and found their son dead in bed.

Oleg then allegedly tried to stab his wife - who was not named - but she managed to lock him in the bathroom and called police.

They had been known as a happy and devoted family until his recent diagnosis with an incurable brain tumour, say reports.

Oleg, a design engineer who formerly served in the Russian navy, spent all his spare time with his son, according to his family.

They went hiking and swimming, and together built outbuildings at their summer house and a kennel for their dog.

8 Ilya, 13, was stabbed to death in his sleep Credit: VK.com

8 Oleg was said to be devoted to his son, seen here with friends Credit: VK.com

8 But the former sailor 'went crazy' after his cancer diagnosis Credit: VK.com

But he reportedly feared that no one could take his place in raising his son when he died.

He is said to have told interrogators his wife could not pay their mortgage and bring up the child without him.

Oleg's mother told the Moskovsky Komsomolets newspaper: "You cannot even imagine how he loved his son.

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“He never punished him. Mostly his mother told him off.

“But after the cancer diagnosis, he went crazy.”

The detained father is undergoing an examination at a secure psychiatric hospital in Moscow.

8 Oleg reportedly feared his wife could not cop after his death Credit: VK.com

8 He told cops he thought his wife could not raise their son alone Credit: VK.com