Yeah, why couldn't he just kill him from way far? He stopped an AdmiraL, so this is just Shanks betting his arm to motivate Luffy to become a Pirate. Shanks is the master chess player in the world of One Piece. We don't know what he's about to do. He even smiled after his arm was bitten. So I think its all his plan to be a part of the downfall of World Government. He has been involved in major plots already. He couldhave arrived at Marineford earlier but let Ace die because he was Luffy's rival to becoming the Pirate King\ Joyboy of his generation. As I type, I get mindblown how this manga has reached 1073 chapters and we still have a lot of plots to be completed.