A demon crept inside the Maxwell Street apartment where Bella Bond died, and kept coming back. 

It wasn’t the little girl, despite what the man charged with her murder allegedly said — that she was a demon, and “it was her time to die.’’ Nor was it her troubled mother, whose bizarre web of lies in the months after her daughter’s death may have ­destroyed the murder case against Michael McCarthy. And it wasn’t McCarthy, either, whose fate a jury will soon decide.

No, these are just people. The demon was coursing through their veins. 

In Suffolk Superior Court on Tuesday, as lawyers delivered their closing arguments, the role that heroin’s destructive power played in Bella Bond’s death came into sharp focus. The rest of McCarthy’s life may hinge on whether jurors believe Rachelle Bond’s shambolic testimony and whether his undeniable weirdness made him capable­ of killing a child because of occult ­beliefs. But supernatural nonsense is not what defined Bella Bond’s sad, short life: That was heroin. 

Just about every adult whose life touched Bella’s in a meaningful way was living with addiction, Assistant District Attorney David Deakin said during his closing argument Tuesday. It was the dark cloud McCarthy said he saw over Rachelle Bond not long after they met, and the one that followed them around the battered blocks of Methadone Mile, where they got high. The toddler in their care should have been their primary concern, but that’s not how heroin works: For those in the throes of addiction, it comes first and there is no second place. 

Addiction does not absolve anyone, and it doesn’t excuse this sad collection of people of whatever involvement they had in the death of a child. It’s not an excuse — but it may be an explanation. We think of heroin’s toll in terms of overdoses and wasted lives spent wandering the streets; we should also think of Bella. Heroin helped kill her, and it helped cover it up. 

On Tuesday, Deakin and defense lawyer Jonathan Shapiro offered directly conflicting accounts of Bella’s death and the months that followed. Each sought to convince jurors, whose lives presumably proceed more or less rationally from one decision to the next, that their version of events made the most sense.

But our everyday notions of what makes sense misunderstand the effects of powerful addiction. Heroin becomes the answer to every question. Those in its grip don’t behave rationally — they don’t show remorse and come up with credible explanations for their actions, they don’t keep their stories straight or behave in ways that betray their guilt or innocence. Heroin is an accessory and an alibi, obscuring everything in incoherence and desperation. 

This entire case, from the day in June 2015 when Bella Bond was discovered in a bag on the rocky Deer Island beach until Tuesday, when jurors began deliberating, has been defined by the opioid epidemic that has ravaged Massachusetts. McCarthy and Bond were fixtures on the sad stretch of Massachusetts Avenue known as Methadone Mile. Addiction treatment workers and the city call it Recovery Road, and plan to open a day shelter to help lure the people populating the streets into treatment. Maybe the next parent who arrives pushing a toddler in a stroller, selling pills to buy heroin as Bond did, will find their way into care. More likely, opioids will keep claiming life after life, not all of them as directly as another overdose. 

In an expertly delivered closing argument, Shapiro took a wrecking ball to the case against McCarthy. He pounded away at Bond’s testimony — littered with contradictions and obvious falsehoods. He showed a selfie she’d taken shortly before she accused McCarthy of murdering her child — she’s grinning and sticking out her tongue. 

Prosecutors, Shapiro said, made a “deal with the devil’’ when they reached a cooperation agreement with Bond, “the only devil in this case,’’ who pleaded guilty to accessory after the fact. 

That’s an appealing notion, in some ways. This case would be easier to swallow if there were a devil we could point to — some kind of demon whose evil acts could be compartmentalized. We could lock up one or both of these people for a good long while, and wash our hands.  

But that’s not what this case is about. The real devil here wormed its way into the veins of nearly everyone whose life touched Bella Bond’s. And no amount of justice will stop that devil from killing again. 

►Lawyer says Rachelle Bond spun lies, B3

Nestor Ramos can be reached at nestor.ramos@globe.com. Follow him on Twitter @NestorARamos.