Herakles killed for a variety of reasons but always as the will of Zeus directed. He was half human and, unlike Mr. Spock, Herakles was very emotional. This made his life vivid and tragic (but with a happy ending).
Nobodys all bad... One notable kindness that Herakles did was to kill the eagle that pecked at Prometheus as he lay bound. Prometheus had stolen fire from Zeus and given it to the shivering mortals. For his transgression, Prometheus was punished by being bound to a lonely rock and tormented by the hungry eagle for countless generations... until the coming of Herakles.
These fun facts were taken from Theogony.