Is there really no chance at redemption ? If you don't put him to death, surely it's because at some point you value human life. Abolishment of death penalty goes with a belief in second chances, in my mind. Can you say than after 25 years, or 50 years, he will still be a threat to society ? Of course if he goes out in 25 years, some families will be pissed. But is the punishment pronounced for them, or for society as a whole ? Will society as a whole mind if he goes out in 25 years ? 50 years ?
Some time ago, I would also have said "life without parole". But really it is amounting at the same as death penalty, and it would be a lot quicker to just execute him now. If one believes in second chances and redeemability, one cannot rule out the possibility of a parole, however distant it may be. I favor a life sentence with no possibility of parole before a certain number of years, e.g 25. I don't know if that exists in MA or the US. It's the worst punishment we have in France (with 22 years instead). With this, believe me, you are already making his whole life miserable and not really worth living, he will probably not have children, not learn a profession, not meet love, not cherish. So I think the goal is achieved.
Being against the death penalty is the belief that the government or law should not have the power to determine who lives or dies. I understand the forgiveness element, but you keep this guy in prison his entire life because if he is released, he is a national security threat.