Ahl Kitaab/mushrik
The fact of the matter is that Mushriks of mecca had openly adopted polytheism as their religion while Jews and Christians hadn’t. Itmam-e-hujjah definitely means they denied the prophet after knowing He was the true Prophet along with the message of that Prophet. However, at the time of giving punishment, just like all other crimes, due regard was given to nature of crime, the condition of perpetrator and conditions under which crime was being committed. Sometimes we end up giving different punishments to the perpetrators of the same crime based on the very factors mentioned above. It is these factors that were being taken into account by God as well while pronouncing their respective punishments. Therefore, we infer that God gave a lesser punishment to Ahl-e-Kitaab because of their lack of open polytheism in comparison to open polytheism by Mushriks. The crime is the same, but it is these factors (i.e. the condition of perpetrator and conditions under which crime is being committed ) which can make the crime of similar nature sometimes eligible for extreme punishment and sometimes for a comparatively lesser punishment.
I thought so because they both (Ahl e Kitab and the hypocrites) had in them the potential – to acknowledge reality and change accordingly because of their closeness to both Islam and Muslims. As compared to the Mushriks.