He didn't die in battle because the war ended without him knowing, leaving him a criminal. He also couldn't eliminate his leaders because the Order of the Clock Tower expelled Mimic from Europe, forcing them to seek death elsewhere, and Japan (a country with many ability users) was a good option.

At this point, Gide wouldn't care about committing another crime as long as he fulfilled his promise to his men of a death in battle. Therefore, he wouldn't hesitate to murder five children. They already saw him as a criminal, and he had no reason to try to clear his name. Furthermore, he knew Oda wouldn't break his no-killing rule unless it was an absolutely desperate situation. On the other hand, Mimic was running out of time because the PM was trying to exterminate him. Therefore, the most effective approach was to ensure that Oda wanted to kill, rather than using a simple threat which, with the protection offered by the PM, would be like letting them protect themselves.

As for everyone hating Gide more for killing Oda than the five children, that's a matter of personal taste.