Hinduism...Why does Krishna advice Pandava's to worship Lord Shiva as redemption of sins?

It is said in Mahabharata, that after the war, when the Pandavas are overcome with guilt of killing so many soldiers, Krishna advices then to worship Lord Shiva at many places on their way to heaven, to get pardon for their sins and clean themselves of Brahmahatya. My question is that if Krishna himself was God, then why would he not pardon the sins of Pandavas than suggesting them to get pardon from Shiva? What is the significance of this episode? Your inputs...