Sakazuki killed multiple marines in Marineford. Garp wants to protect the young marines from threats inside and out, yes he 100% would’ve killed Sakazuki who was killing marines who didn’t want to fight a pointless war.
Bro, really think about what you´re saying for a moment. Like, take a minute.
Sakazuki killing the deserters isn´t really implied to be something the characters in the story all knew about, the readers know that, but Garp doesn´t inherently know about that, and if he did, he would know that it´s probably some sort of Marine practice that someone like Sakazuki takes seriously. The only relevant player here is Ace and Koby. Only Koby would be Sakazuki´s original idea, and again, this is part of his flawed consistency, it´s not the same as Imu killing anyone near him.
And God valley and Marineford aren´t particularly comparable.
1.Roger and Rocks were not going out of their way to be threats to the Marines on GV. They believed they were after a girl and that´s it. Teach was planning on sinking Marineford, including *all* the marines, who would die. This is obviously the primary issue
2. Sakazuki isn´t doing what Imuturn is doing, Imuturn was pretty much willing to kill everyone on GV
3. Garp goes on to leave Saturn alone, so clearly his motive isn´t revenge, but guranteeing the safety of Marines as a whole, a goal Sakazuki wouldn´t be against.
But anyway, even if Garp did go against the Marines, it would not actually be hard at all for Sengoku+the three admirals to kill him. Like, Kizaru could do what Shiryu did and Kuzan can do the rest, all before he can even touch Sakazuki. This isn´t overly complicated, and the Marines would still have time and resources to fight the Blackbeard pirates and Red Hair pirates.