In one incident, at a different party, he pulled a knife on another teenager, who ran away.
In another incident, he went armed with a machete to meet a girl.
Derrick-Hardie split from his girlfriend, who has name suppression, after she found photos of a previous partner on his computer.
During the argument, Derrick-Hardie said of his previous lover: "It's all [her] fault. I'm going to find her and slice her throat."
The following week, Derrick-Hardie went to a birthday party in Pakuranga. His ex-girlfriend was there, as was Mr Rosillo, 18, known to his friend as WoF because of his love of cars.
The court heard how Mr Rosillo and Derrick-Hardie's ex-girlfriend were chatting and flirting, when Derrick-Hardie approached them and asked the woman to leave with him.
Eventually she agreed and the two went outside. But they started arguing and were soon yelling at each other.
Mr Rosillo approached the pair and tried to defuse the situation, but Derrick-Hardie told him to leave so he could speak to his girlfriend.
The woman said she was not Derrick-Hardie's girlfriend. Derrick-Hardie accused her of being unfaithful.
She responded by kissing Mr Rosillo, who kissed her back.
Derrick-Hardie grabbed Mr Rosillo by the neck and pulled him in close. With his other hand he took a knife from his trouser pocket and stabbed Mr Rosillo in the head, chest and abdomen.
Mr Rosillo died on the street, despite his friends' desperate attempts to revive him.
Derrick-Hardie ran away, and within minutes began sending text messages to friends. He told one friend at the party: "Bro. You didn't see me tonite."
Police arrested the teenager in a room at the Heritage Auckland hotel.
But the arrest did not stop him calling his ex-girlfriend, who at the time was giving a statement to police.
Derrick-Hardie told her that she had seen nothing and the phone call never happened.
He has been remanded in custody, and will be sentenced in September.
Words of a killer
April 16, 2011
Online conversation between Theodore Derrick-Hardie and his then-girlfriend:
Only reason you should worry near me, is if im worried cause then someones gonna die ... or get eaten ... or im gonna kill someone or possible eat them ... or myself ... Depends if im drinking gas again.
April 25, 2011
Online conversation between Derrick-Hardie and his then-girlfriend (he had just been talking about joining the army):
Derrick-Hardie: But its my calling
Girlfriend: how do you know!?
Derrick-Hardie: Because im a dangerous person, this way im using my talents and mind to serve and protect, because if I didn't do this
Girlfriend: Your dangerous!? Haha don't make me laugh! I know you're the kindest person there is
Derrick-Hardie: I would be selln meth again stabbing n****rs ... I know I've changed a lot. And danger is not in the hands of the beholder. Its in the heart and mind ... When you think about murdering someone because they ... Mocked you
February 25, 2011
In one online conversation with a woman, he talked about men being "made like robots".
"We are made to do what were born ... kill or be killed. Give or take if we in kill mode."
He went on to write in text speak: "We made fast choices ... quicklyjsut do this or that ... and dnt think about it ... jsut stab stab stab."
