The blueish green color was probably because he appears to have been going into shed at the time of death.

This snake looks as if he were mauled. Do you have a cat? had he recently escaped his enclosure? He also looks a bit on the thin side on some of your pictures, but not all. This could be because he was dead and deflated, or because he was underfed or was harboring parasites. He also has some scale rot on his ventral scales. Scale rot is usually a result of poor husbandry. Was this your first garter? If so, you may have made some beginner's msitakes. Any amount of humidity in a garter's cage can be disastrous for him.

How long did you have the snake, and how where you keeping him? Did he have a nice dry enclosure with a large clean bowl of water? A thermal gradient? How often did you feed him and what was he getting? If you used fish, what kind of fish? If you can tell me these things I may be able to further help you out as to what brought about the demise of your little guy

Were you possibly keeping him on a bark substrate? If he were to ingest a piece of bark and then try to use his muscles to work it down, he could have punctured himself from the inside out.