I do not just forgive someone for doing 1 good thing alright. But you do have to weigh the things Luffy has done. Yes he did release those prisoners and that is horrible however he has done a lot of good. He has saved multiple kingdoms such as Alabast, Dressrosa, Drum Island, Skypeia and so on. On top of this he has done a lot of other good stuff throughout the story. I am not saying he has never done anything bad however I do think the stuff he has done right far outweighs the very few bad things he has done thus far in the story and if he were executed purely for defying the marines then a lot of stuff he could do in the future and all the lives he could save will not be saved and will not happen.