Makes me wonder if Marco regrets following Whitebeard’s order to stand back even though Blackbeard was attacking him. Along with Garp and Luffy, Marco surely took Ace’s death pretty hard as well, along with also losing Thatch and Whitebeard, a father figure he had known for like 30 years of his life, and being betrayed by Blackbeard in thinking that he was a brotherly figure when it had later turned out that Blackbeard was someone who might as well have easily left all the Whitebeard Pirates for dead if it had come to that.
I’m imagine Marco must have been thinking of a lot of things in light of all that and probably ended up deciding that it’s honestly fine for him to live as he feels he should as much as he respects Whitebeard and his words otherwise.