There are four capital punishments (Skilah, Sreifah, Hereg and Chenek) and there seems to be a hierarchy in terms of severity (heavy to light) but is there any discussion of how the specific crime merited a particular death or why a specific death method is appropriate for a crime? Is the death sentence a natural consequence (which would make it inherently tied in its method to the crime) or a punishment (so the severity matters but the particular method doesn't)?
If the goal is punishment then the penalty is simply death - how can we justify any method of killing that is more or less severe? Shouldn't the end result, death, be the point so we should use a method like we would in shechita, to get to the desired end as quickly (and painlessly) as possible?