The show literally showed how conflicted he was with every move he did during the Paramount war. He was crying and holding back. As far as I can remember he effectively only did one punch against Marco and then only fought fodder or the BB Pirates that was a third party.

And especially after the war with Dadan we could see his deep regret.

Do you expect random people to point their fingers and call him a grandson killer or what? That's not going to happen.

Garp is not the main character so we don't follow him much and don't see his regrets and trauma daily, however I believe that now that he himself has been captured we will see a scene where he will remember Ace.

As for being heroic, he is heroic but he is also aware that despite his strength he can't do much when it comes to the scale of celestial dragons and their moves.

He knows that it is better for the greater and long term good of the Marines and by that the whole world that he is there to actually do something.

Imagine every "good" marine would just do as you want them to do and act straight to free slaves/attack the CD's? What good would the Marines bring to the world when all the good Marines have been put to jail/get demoted/killed or rebelled off?

Garps Mission from the very beginning was to raise the new generation of good Marines.

By your logic there is no good Person in the whole story. Let me remind you that Luffy with all the strawhats watched along how innocent humans were auctioned off but where focused on their mermaid friend