But during closing arguments on Tuesday, Suffolk County prosecutors put the focus on what they say is another culprit: heroin addiction.
"(It's) primarily heroin that led us all here," assistant district attorney David Deakin said. "If there weren't heroin in this case, there wouldn't be a case."
McCarthy, 37, has pleaded not guilty to first-degree murder
. Prosecutors say he killed his then-girlfriend's daughter, put her in a trash bag and dumped her in the waters of Boston Harbor. His defense attorneys have argued that the accusation is largely based on the words of the girl's mother, Rachelle Bond, who blamed McCarthy to cover up her own role in Bella's death.
Both McCarthy and Bond, as well as every important adult in Bella's life, suffered from serious drug and heroin addiction, prosecutors said. Drugs consumed them, altered their decision-making and destroyed their lives in a way that prosecutors admitted is difficult for non-addicts to understand.
"The result is you get a sort of fog, a haze of drug addiction," Deakin said in closing arguments. "You get a sense that decisions were made that people who are not addicted to drugs, not addicts, might never make.
"That haze of drug addiction permeates this case," he said.
'I know that lifestyle'
From the beginning, Bella was surrounded by addiction.
She was conceived while her parents, Rachelle Bond and Joseph Amoroso, were living in a tent at Occupy Boston, Amoroso testified. They were both homeless and using crack cocaine at the time, he said.
Addiction was not a new issue for Rachelle Bond; her criminal history included arrests for heroin possession and prostitution, CNN affiliate WCVB has reported. In addition, she had two other children
who were taken away by the Department of Children and Families (DCF).
"We know that all her life she had a struggle with drug addiction and mental health issues," Assistant District Attorney David Deakin said.
Before Bella was born, Amoroso moved away, but he would occasionally speak with his daughter over the phone.
Friends and neighbors testified that Bond was a loving mother to Bella. The mother-daughter pair held hands and said grace before meals, played out in the yard, and Bond was kind to Bella, prosecutors said.
"We all know she wasn't a great mother," Deakin said. "But within her limitations, she was trying."
In one call, Rachelle Bond was slurring her words, so he called DCF to report that Bella was in an unsafe home, he testified.
Amoroso later returned to Boston in an attempt to meet Bella in September 2015. He knocked on the door of Bond's home and immediately recognized that she was in the throes of addiction. "She looked like she was strung out," he testified. "I know that lifestyle and I could see it from a mile away."
Though Amoroso didn't know it, Bella had already been dead for months at that time.
'The evil of heroin'
The impact of heroin in the case is most evident in the few months after Bella's death.
Bella's unidentified body washed ashore in June 2015 off Deer Island near Boston. Authorities launched an extensive investigation to try to identify the body of "Baby Doe" and commissioned a forensic artist to draw a composite image of the girl. Thousands of tips poured in, but her identity remained a mystery for months.
Yet while the public searched everywhere for the child's identity, McCarthy and Bond were living together in the midst of a heroin binge.
Bond did not tell police that her daughter had been killed, and instead used heroin as a way to deaden the pain she felt from Bella's death, prosecutors said. They continued using every day after that, too.
"That's the evil of heroin," Deakin said. "Rather than doing something about her situation, rather than doing what was right, she did what was easy, she did what was painless -- she went to the drugs."
Bond pleaded guilty to accessory after the fact to murder. She will be sentenced to time served and probation for her testimony in the case.
The investigation broke open in September 2015 after Rachelle Bond told a friend, Michael Sprinsky, that Bella was actually dead.
"Please say it ain't so, Mike," Michael Sprinsky texted McCarthy afterward. "She said you killed Bella, bro."
McCarthy responded by denying the accusation and then taking aim at Bond's history of drug use. "Wow, you listen to a cracked out hooker," he responded.