Maybe fitting doesn't require resembling. I'm inclined to think that it gives people an emotion. sense of fittingness if there is a similarity between the offense and the crime, but retributive theory just says you're supposed to deprive the offender of happiness in a measure that is calibrated to the wrongness of the act. There is nothing in that that requires that the form of the punishment resemble the offense.
When a murderer is executed, the death of the murderer differs in many ways from the death of the victim. The state makes sure the death is relatively free of physical pain, something murderers don't care about. The person executed has an execution date set on the calendar for years prior, the victim doesn't. So the murderer's experience differs in many ways from that of the victim.
Everyone is under a death sentence, anyway.