[SHINOBI03]

Of course we learn that not all monsters are the same. You have monsters that can be reasoned with and if they get what they want without hurting anyone then they go free alive and a lot of the conflict is usually by a misunderstanding from one or two sides and this will show a lot in later installments.

Other monsters are big animals that don't intent on hurting anyone but their existence on itself endangers everything around them and if there's no pacifist alternate then Ultraman and his team have no other option other than to terminate them.

Then you have monster that were victims of certain circumstances where they were results of mad experiments or horrific mutations that depending on the situation could or could not be reasoned with. But they're mostly killed to end their suffering when nothing could be done to save them.

And lastly evil monsters that are irredeemably evil who love to cause pain and suffering to anything they see, be it someone like Red King or Geronimo who are evil on their own free will or space monsters sent by aliens to conquer and destroy other planets and those are the type they choose to kill when all negotiations fail.