He was saying that it was up to him to make things better for mankind. He was trusted and was always there to make things better. He was the one to help you forgive those who trespassed you and used you.
I think Marley means that to him it was not just business he felt like it was his duty to help mankind because when e states that "BUSINESS! Mankind is my business" he was trying to say that to him it didn't feel like a job it felt like his duty or his was of paying back to the world. He didn't feel like he had to but he wanted to. Hope this makes sense...
