You are absolutely correct that Justice demands the punishment fits the crime. In human terms, that's why there are human laws that deal with the crimes of man. However, human punishment has no bearing on the soul of the individual. Human justice cannot punish the soul, only the body. Therefore, human justice is incomplete.
The reason eternal torment is necessary is for the soul. When we sin, not only is it usually a mental and/or physical act of defiance towards God. It is also a spiritual act of defiance towards God. It's "I don't need you to tell me how to live my life, my soul is fine on my own". That's the sin Adam & Eve committed against God in the Garden and why the spiritually died that day. At the end of life, the soul lives on and it would seem that as it was in physical life, it is in physical death. If it accepted Christ's gift of salvation, then it is spiritually reborn and lives forever, glorifying Christ. If it did not accept Christ's gift of salvation, then it is spiritually dead and forever curses Christ.