You guys forget about basic war laws and psychology of warfare. There's a reason why it is considered okay to kill combatants while in conflict, but it is strictly prohibited to not only torture, but also treating POWs inhumanly.
War is an extreme extension of diplomacy. By exchanging damage numbers in both troops and property sense, countries are resolving their disputes, ultimately proving to each other who is more 'right'. Human resources that are no longer capable of changing these numbers (KIA, MIA and medevaced) are therefore no longer can be considered elements in this game. Because of this, they become technically uninvolved for the time being and therefore the agreements regarding damaging warring countries' resources (both human and material) no longer apply to them.
Simply put, conflicts such as wars are resolved by damaging, destroying each other's resources. Torture doesn't really help nearly as much as killing, and also is considered much more inhuman and uncivilized than simply killing people. Because of this, inhuman treatment, including torture, of noncombatants (i.e. anyone who can no longer continue carrying military actions) is condemned and prohibited by a number of laws and agreements worldwide.
That is to say, while a combatant is still engaged in action, that is, before they are killed or captured, it can be justified to torture them in order to gather military intelligence. Usually, though, such actions are carried out by special forces and exist in the grey area of legality.
In the case of this manhwa, since MC isn't originally a member of armed forces, he could feel tolerant towards killing someone in order to survive, but such things as torture would seem barbaric and horrible to him. All in all, the idea that torture is more humane than killing is flawed at its core because of the sheer amount of suffering that subject is receiving. The is a reason why in media we often see victims of torture asking to kill them instead.