It's really understandable in a sense. It just we people think that's wrong because it's against our moral value.
Think people with fantasy power as a country with their military power in our world.
When some country grow it's military more and more, what do you think the current superpower will do against them? Cooperate with them to create better and safe world?
No, this is so incredibly short-sighted, anybody who agreed to it is unfit to run a bakery, much less a country.
1) When summoning a hero, you are asking gods to send their chosen champion and bestow him with divine gifts. How you treat the hero, will affect your standing with gods. They can deny your country their blessings (like breathable air), or they can resurrect the hero on the spot as unstoppable rampaging dullahan.
Or they can make you suffer more than that, like sending an epidemic of painful sickness and withdrawing your church's healing powers.
2) You will need the hero's strength. Evil is never fully defeated.
3) You clearly have a princess who is suitable for marrying him, turning him into a valuable asset of your kingdom. Why squander this chance?
4) What would you do if the hero isn't such a goody-two-shoes and will decide to slaughter you all in the face of death?
5) What would you do if the hero is such a goody-two-shoes, and his death will upset the local karmic balance?
6) The next hero who is summoned to your world will learn about that. And any brave people of your world will learn about that too. And so, the monarchy is doomed, because they will find a way to avoid it happening again.
The bottom line is, when you live in a fantasy world, being good is essential to survival. This isn't reality. Randomly doing evil will get you killed. Or, if you're really unlucky, it will not kill you, but make you suffer for eternity.
P.S. In the real world, USA is incredibly short-sighted and evil. Most other countries have no problems cooperating.
For most strong countries, like Russia and China, their military power exists to protect themselves and their friends.