Moral responsibility is related to but not the same thing as moral obligation, and it's completely possible for a utilitarian to say one is morally forbidden to be a bystander and let a murder happen while admitting that doing so doesn't make you responsible for it. This is because responsibility is about causation and obligation is about what one ought to do. Murderers cause murders and are therefore responsible for them, while bystanders are innocent. The utilitarian should say not that the bystander is as morally responsible as the murderer (because they aren't), but that moral responsibility isn't what ultimately matters.