Is Eternal jahanam so severe for the purpose of deterring people from sin, or is it strictly proportional retribution for the "crime" of disbelief?

It says in Al Hidaya By Burhan ad Din al Farghani al Marghinani "Qisas (retaliation) is not called hadd as it is a right of the individual, nor is ta'zir called hadd as it is not predetermined. The primary purpose in promulgating it as law is deterrence from acts that are harmful for subjects"... "Purification (from sin) is not the primary purpose in hadd, on the evidence that it is ordained for the unbeliever as well" Source: book chapter 97, book 11 , page 207 https://archive.org/details/al-hidaya-in-english/page/n457/mode/2up From what I understand, Hadd punishments are very severe, and as it suggests, the purpose is deterrence rather than equaling the crime done. So, in islam Eternal Jahanam is very severe. Like Hadd, is that because the crime of disbelief is so severe or it is for the purpose deference? Please could you correct any misunderstanding Thanks