A person convicted of manslaughter (either degree) cannot be given a sentence of probation. Manslaughter in the second degree is a violent class C felony and carries a mandatory minimum sentence of 3-1/2 years in prison. Manslaughter in the first degree (which is what the defendant is guilty of) is a violent class B felony and carries a mandatory minimum sentence of five years in prison. The only violent felony class that can get probation as a sentence are class E felonies and the only homicide charge that is a violent class E felony is criminally negligent homicide, which clearly does not apply in this case.
Considering that there was a decent possibility that the defendant was suffering from an extreme emotional disturbance ADA Cabot should have given the jury a choice between finding him guilty of murder in the second degree or manslaughter in the first degree. Cabot was concerned that the jury would feel sympathy for the defendant and find him not guilty of murder because of his emotional pain and grief, which is why it would have made sense to include the lesser charge of manslaughter.