The Sloth death isn't exactly clear...generally we're supposed to assume that deaths to the victims are supposed to be a sort of...'poetic' justice and a part of John Doe's larger spectacle and how each represents their respective sin.
We don't know very much with him, besides he was forced on drugs and rotted away in bed for a year or something. He should represent slothness (even this sin as a description isn't exactly clear)...whatever that means. And I assume in this particular case it should be the laziness. The complete and utter lack of doing nothing and wasting all possibilities on nothing.
Doe shows this by having a person 'presumably' be able to escape, but does not and simple wastes his body and brain. But Se7en has several missing links....this particular one is probably one of them.
However I would say yes, don't think about it so much...the film is 25 years old now...there are bound to be some plot holes. :)