Even to describe the Ancient Mariner's act in killing the albatross as a crime is to say both that he should have been punished and that the punishment should have been proportionate to the crime. Neither of these points are suggested in the poem. The Ancient Mariner's shipmates vacillate according to circumstances as to whether he did anything wrong or not. First, they say that shooting the albatross was "a hellish thing" for which they will all be punished. Then, when the sun rises "like God's own head" they change their minds and say he was right to kill the bird.
This uncertainty and the language in which it is described place the Ancient Mariner's action in the realm not of crime but of sin. There is no requirement for the punishment of sin to be proportionate or, at least, to appear proportionate to the sinner. God can forgive a murderer or punish the entire human race because one man stole an apple. The punishment of the Ancient Mariner certainly seems disproportionate, as he suffers horror after horror for shooting a bird, hardly an uncommon act and difficult to distinguish in moral terms from catching fish.
Sin, however, is largely a matter of motive, and the Ancient Mariner's motives are shrouded in mystery for the reader. He does not say why he kills the albatross, and only God can determine what is in his heart. If God is just, Coleridge and the Ancient Mariner both seem to be saying, then his punishment must have been just as well, even if neither he nor the reader understands why.