If he had intended to kill him the charge would have been Murder, not manslaughter.
Involuntary manslaughter
Involuntary manslaughter is the unlawful killing of a human being without malice aforethought. It is distinguished from voluntary manslaughter by the absence of intention. It is normally divided into two categories; constructive manslaughter and criminally negligent manslaughter.
The intention mentioned probably means that, a charge of manslaughter would be brought if the person intended to cause harm (by an unlawful act) but not to kill.. So if I punched a guy on the nose, unlawful act, and he fell, hit his head and died, charge would be manslaughter.
