An engineer who allegedly murdered his girlfriend told the High Court yesterday that knowing her was like winning the Mark Six.

Ng Yan-kin, 25, also said he hired a prostitute to learn about sex and was suicidal after a friend tried to cheat him of HK$60,000 by lying he had AIDS.

Ng pleaded not guilty to murdering nursing student Annie Li Sin-heng, 20, by stabbing her to death on the upper deck of a CityBus in September 2017.

He admitted manslaughter, but the prosecution rejected the idea he committed the crime due to mental instability.

Ng told Justice Audrey Campbell-Moffat he was bullied in primary school after becoming friends with a new immigrant. And he was laughed at by secondary classmates after talking about sex.

Ng said he met his first girlfriend through WeChat in 2014, but "I was not handsome or rich, so I was afraid she would leave me."

He also worried she would laugh at him as he had never had sex. So he took the suggestion of a friend, Alex, to hire a prostitute to learn about sex.

But a condom slipped off during intercourse with the prostitute, and Ng feared he would catch a sexually transmitted disease. So he gave blood and urine samples to Alex for tests.

Ng was told by Alex later he had AIDS. But Alex said he could help him buy US medication that could kill 90 percent of the virus and asked for HK$60,000.

Ng said he paid Alex HK$30,000 for a few pills, but he thought of suicide as he knew there is no cure for AIDS.

Although Ng later realized Alex had lied to him and he was not infected, he would still get tested from time to time.

Ng said he met Li on the internet in July 2016 and they had sex after knowing each other for two weeks.

Ng described Li as independent but caring, and he wanted to spend his life with her. "[Knowing her] was like winning the first prize in the Mark Six," he said, but he was worried Li would break up with him and checked on her constantly.

He checked on Li's whereabouts through apps and made her send him a photo every hour to show her location.

The trial continues today.

Shocking tale led to student nurse's killing