Turner also admitted to jurors the messages "upset" him despite confessing he had slept with "a number of other girls" while seeing Emily Longley, 17.
He told the court he was "upset and surprised" after reading her private Facebook messages in which she commented "how sexy another guy was."
Turner wore a smart blue suit, white shirt and navy tie as he took the stand to give evidence.
He said: "Emily had left her Facebook account logged onto my laptop and out of curiosity I decided to read her emails.
"I found out some things I didn't really know, they were just things about our relationship, quite derogatory comments about me to some friends.
"I felt upset and surprised. The perception she had given her friends wasn't the same perception she had given me.
"There were a few things in the email about her ex-boyfriend and talking to other boys.
"They were quite flirtatious and she had been commenting how sexy another guy was.
"There was also an email from Lewis Hillman who asked if he could pick up his Armani watch from her bedroom."
He told the court he rang Emily to confront her on April 30 last year but "she denied what I was questioning her about."
He added: "I wasn't faithful to Emily and during my relationship with her I had sex with a number of other girls.
"There were no lasting relationships, they were brief one night stands."
TURNER EXPLAINS HAMMER POSSESSION
Turner admitted going to Klute nightclub in Bournemouth on May 5 with a lump hammer in his trousers after he heard she would be there with another man.
He said: "I was in quite a huff and believed Emily was inside with someone called Louie Powell.
"I walked in with the mallet in my trousers but Emily wasn't there.
"I had a friendly drink with Louie but I showed the mallet to him through my shirt and told him I had a friend in the club with a gun.
"Then afterwards I told my friends I had found Emily in the club and killed her with the mallet.
"It was a prank and afterwards I told them the truth."
'I ADMIT SAYING I WOULD KILL HER'
Earlier jurors heard how, while on a trip to the Isle of Man in March last year, Emily wrote in a letter to Mr Turner 'don't say you will kill me'.
In examination Anthony Donne QC said: "Had you said that you will kill her?"
Turner said: "I admit saying I would kill her but I did not literally mean that.
"I think I said that to her about 10 or 15 times."
When asked by Mr Donne whether Emily had said it to him as well he said: "Yes all the time."
Mr Turner came up with the idea of the letters to save arguments and let Emily tell him what was upsetting her and how he could change.
She also asked him 'to stop talking about your ex-girlfriend', 'to stop being so constantly aggressive' and 'to be more cool'.
Mr Turner also said she started and ended the letter by saying 'I love you'.
ANGER OVER MODELLING SESSIONS
Earlier in the day Turner expressed anger over photographic modelling sessions she was doing weeks before her death.
Yesterday, jurors in Winchester Crown Court were read an exchange between Turner and Emma Ross, a mutual friend of the couple, on the social networking site Facebook.
Turner said: "I don't know who organised the last [photographic] shoot but I don't want Emily going out with pervy lads or my enemies because I will f***ing kill them.
"Stop encouraging Emily to do these shoots, it's unacceptable to organise these shoots.
"Put yourself in my position, it's not nice to do these kind of shoots."
When Emma did not reply he sent her another message.
He said: "Emma stop trying to f*** up my relationship. It's making Emily look like a whore.
"Show me some f***ing respect you little sket. I know what you're like, you're a bad influence on Emily.
"I don't want her going to these shoots ..."
Emma replied: "F*** you, you possessive dweeb, I know more about you than you know.
"Are you trying to scare me with your little bad boy acts? It doesn't scare me.
"Bad influence? Not being funny but Emily has her own mind, it's down to her. Speak to her if you have issues."
Jurors have been told Turner had become increasingly suspicious that Emily had been having affairs during their four month relationship.
Turner has been charged with murder, and perverting the course of justice. His parents, Leigh, 54, and Anita, 51, have been charged with perverting the course of justice.
Emily was born in Britain, but her family emigrated to New Zealand when she was nine and she was raised in Auckland.
She had returned to live with her grandparents in Bournemouth.
