A large portion of Trump's base has a raging hard-on for anything military related, and worships anyone considered "elite", such as SEALs, Delta Force, Rangers, etc. They will be quick to defend soldiers accused of human rights violations, saything things like " I detest the idea that we send our soldiers to fight and kill the most dangerous and deadly people on the planet and then second guess how they do it," or "Please go become a Navy seal and fight these terrorists. Show us how you can do it better before you call these guys criminals," or "Thats just how some of our military cope with the ugliness and nature of warfare. The PC culture, news media, and rules of engagement the military adopted under Obama's regime has hamstrung our military. Equating the US military with Isis and what they have done is disgusting," or "We do not need to follow all these UN rules during war. That is exactly what makes us lose wars and why Vietnam was lost. That is why the British lost the war and America was formed because we did not follow the rules of engagement. I am with Trump!"
These people have also created an image in their heads that Trump ("I did this for the war-fighters.") understands the military better than his commanders, and is more patriotic than anyone else, saying things like "Thank you President Trump for standing up for the men and women in the trenches, on the hill, dodging bullets, jumping out of helicopters under fire not sitting behind desks shuffling papers and playing video games," and "Than God that we finally have a Comander in Chief who is actually willing to lead and make decisions!" and "Thank you, President Trump, for protecting our military personel against the unfair attacks from rabid pacifists and those with no idea what it is like in a war zone!" Ironically, these same people fail to remember or outright ignore the fact that Trump himself has no idea what it is like in a war zone due to getting a medical waiver for bone spurs so he could avoid being sent to Vietnam.
These are the same people who, when news of a unarmed black man being shot by a police officer comes out and don't bother to gather all the facts, say things like "He shouldn't have resisted," or "he must have been doing something wrong, or the officer wouldn't have needed to shoot him." The police and the military are infallible in their eyes, and anyone who questions the motives or actions of someone in those groups they immediately dismiss the charges as "PC," "Liberal," or "Unpatriotic."
Trump pardoned Gallagher and the other soldiers to please this part of his base. It cements this image of a Commander in Chief who has the backs of the troops and protects them from the ignorant brass and bureaucrats. If our soldiers happen to be a little bloodthirsty well that is just part of the job.
When I was in officer training for the Air Force, we learned all about My Lai and Lt. Calley. The military does not want its soldiers becoming bloodthirsty monsters. The moment we allow ourselves to do that we lose our moral authority. If you don't see the problem with killing unarmed non-combatants then I suggest you read about My Lai, the Bataan Death March, as well as read Ordinary Men. We have a moral obligation to be better, to be above these kinds of actions, and Trump is not making America great by pardoning people convicted of war crimes.