A police informant who says he drove Jimmy Melvin Jr. to the home where Terry Marriott was shot to death began a romantic relationship with the victim's widow within two months of the homicide, a Halifax jury heard Wednesday.

Derek MacPhee, a career criminal, is the Crown's star witness in Melvin's first-degree murder trial, which began this week in Nova Scotia Supreme Court. Marriott, 34, was killed while taking a nap at a friend's house in Harrietsfield, N.S., on Feb. 20, 2009.

On Wednesday, MacPhee faced cross-examination from Melvin's lawyer, Pat MacEwen.

"I'm going to suggest to you that you actually had this relationship with Mr. Marriott's wife before he was killed," MacEwen said to MacPhee.

MacPhee acknowledged he began a relationship with Nadine Fleming after Marriott's death, but said they were only friends when he was alive.

Cocaine

MacPhee admitted to the court that after the shooting and before the police were called, he searched Marriott's pockets looking for cocaine he had given him earlier that day to sell to a fisherman.

MacPhee testified he found 4½ grams of the 15 grams of cocaine he had given Marriott and $15. He hid the drugs in a fish tank.

"I still had to pay that bill after he died," MacPhee said.

MacPhee has been given immunity from prosecution for his role in the murder and in a violent home invasion in exchange for his testimony against Melvin. In all, he's had more than 30 charges wiped out for his co-operation.

MacPhee was kicked out of a federal witness protection program because he committed more crimes after signing his agreement. However, he signed another agreement with Halifax Regional Police in which he's being paid for his testimony and to move out of Halifax.

Exchanged guns

MacPhee testified he had earlier driven Melvin through the woods on a four-wheeler to the home of a man named John Lively after MacPhee told Melvin that Marriott was there. They went there with two loaded guns.

MacPhee said he parked across from the home and down the street when Melvin went inside. A short time later, he heard five loud bangs. Before police arrived, MacPhee tried to get rid of Melvin's footprints.

MacPhee also testified that shortly before the murder he had exchanged his .25-calibre handgun with Marriott's .38-calibre handgun, which people had seen MacPhee carrying.

To deflect any attention from his own involvement in the crime, when people asked about the gun, MacPhee told them he had given Marriott the .38 back and said that he "hoped Terry didn't get killed with his own gun."

Desire to kill Marriott

MacPhee earlier testified that Melvin mentioned to him several times that he wanted to kill Marriott.

During cross-examination, MacEwen said it didn't make sense that MacPhee told Melvin where Marriott was when he knew Melvin's intentions. MacPhee replied:

"I understand but there's only so many times, how many times can you try to save a guy's life?"

While Melvin didn't "put a gun" to his head and force him to drive, Melvin "was persuasive," MacPhee said.

MacPhee also admitted that after the murder he gave his shoes to a friend to toss and washed his clothes to get rid of any gun residue that might have been allegedly transferred from Melvin.

Judge warns jury about MacPhee's testimony

Following MacPhee's testimony, Justice Jamie Campbell warned the jury about MacPhee's evidence when they decide the case.

The judge reminded the jury that MacPhee testified in this matter with an assurance that he would not be charged for a range of other offences, including his involvement in the killing, and has also received money in return for his testimony.

MacPhee, the judge said, is not just a career criminal.

"He has numerous breaches of court orders and has stolen upwards of 300 cars over his life," Campbell said.

Witness has lied to police, court

On top of being convicted of possessing counterfeit money, MacPhee has repeatedly lied to the police and also lied under oath in court.

The judge told the jury it is dangerous to rely on MacPhee's evidence unless it can be confirmed by another witness or other testimony.

"There's a danger in convicting a person for murder based upon the unconfirmed evidence of someone like Derek MacPhee, though it is possible to do so if you're satisfied that it's true," Campbell said.

The trial continues Monday.