In a lot of mangas, it bothers me that people who don't seem to be psychopaths before reincarnating think of the new world as just a game. Thinking of people as NPCs or Mobs, wanting to be a hero at the cost of reputation, or just over all being a huge douche.
In most isekai manga, the new world has game mechanics. A game UI of some sort extremely often, though sometimes it only exists for the isekaied person(s). Experience points got from slaying enemies, skills with numerical progress values, attributes with numerical values, far, far beyond checking how many kilograms you can lift. If the new world has such a system copied straight from the most basic RPGs, then it's not unreasonable for the summoned people to treat the place like a game, to varying degrees. Only interactions with local friendly folks will alter that perception. However, if the locals are hostile or otherwise nasty people, like in this story the human kingdom is dreaming of destroying the countries of all the other races, then treating the place like a game becomes all the more plausible.
Even in a more benign setting the locals can also treat their world in a game-esque manner, because they as well will make quick progress toward becoming powerful by slaying a maximum amount of enemies, typically monsters.