Complicity is a concept that can be abused by prosecutors. The law sees it as a separate offense because it's really helping someone avoid arrest or escape punishment more than helping someone commit a crime.
Accessories always have a claim to less punishment.
Their participation is what creates the liability as if they had committed the crime alone. The relationship is what creates the liability. It dispenses with the element of actus reus in the same way strict liability dispenses with the element of mens rea. One night after going to bed, David choked his wife to death for refusing to make love to him. He dragged the body down to the first floor to dismember it in the bathtub.