A twisted young man has been found guilty of murdering a chef with a spear gun and a carving knife because the victim's sister dumped him.

Andrew Morris, 30, was a brilliant Cambridge trained mathematician who went on to be a £120,000 a year actuary with a golden future. But he looked down on those he thought less intelligent - and had a desperate need to hurt anyone who crossed him.

When Michelle Stangroom broke up with him, he blamed her brother Henry, 21, from Odiham, a talented chef working at the Criterion restaurant who also had a bright career ahead of him.

Psychiatrists who examined Morris believe he was arrogant and selfish and may have wanted to hurt his former partner by murdering the young cook. On October 18 last year before he killed Mr Stangroom, he squeezed Michelle's pet hamster to death - and even filmed himself doing it.

Morris then fired a diver's harpoon at Mr Stangroom as he slept, but the dart only caused a glancing blow to the victim's head. Mr Stangroom awoke up in terror and Morris repeatedly stabbed him in the chest with a kitchen knife before turning the speargun on himself.

Police found Morris lying in the bath 18 hours later and he had apparently attempted to slash his wrists and the harpoon was sticking from his neck. Prosecutors claimed he did not intend to kill himself and knew he would be found by an estate agent who was due to call at the flat.

Morris claimed he acted in self-defence and that he was attacked by Mr Stangroom who had accused him of stalking his sister. He also said he was mentally ill at the time, but the Old Bailey jury found him guilty of murder on Friday May 23.

Before he killed Mr Strangroom, Morris researched 'horror speargun accident', 'speargun in heart' and 'how to kill someone in their sleep,' on the internet. He also searched 'murder mental illness,' 'sentence for murder in UK' and 'murder and mental illness sentencing'.

Morris looked stunned when the verdict was delivered, and slumped back into his chair letting out short sharp breaths as he was told he will be jailed for life.           

Henry's parents broke down in tears when the jury of six women and five men delivered the verdict. Judge Wendy Joseph told the jury 'emotions in the courtroom have run high' during the trial.

Morris had been violent towards previous lovers and became a father when he got another girlfriend Laura Teague pregnant, but abandoned her and his son and began his affair with Michelle, who he met when he was working for a company called Pensions First.

They moved into the flat in Lavender Sweep, Battersea in September 2012 and her brother moved in with them three months later. All three were drug users and took cocaine and nitrous oxide.

Morris was also suffering from depression and panic attacks and began taking more and more time off sick. By May of 2012 he was spending most of his time lying around the flat watching daytime TV, snorting cocaine and eating food from the saucepan.

After attacking Michelle she moved out and told him she would not see him again unless he dealt with his problems. She also insisted he should delete the dozens of sex tapes he had made of her. Morris was so obsessed with taking pictures of Michelle he would even film her getting dressed in the morning.

Top psychiatrist Dr Philip Joseph said: "When he feels rejected or jealous or his emotional needs are not met, he can respond with petulant, immature or aggressive behaviour."

He said Morris was sane, but displayed 'arrogance and narcissism and believes himself to be superior to others.'

The judge adjourned sentencing until May 30.