Krishna and Shiva were at first friends. Krishna first met Shiva as a man who taught the Yoga and was a man in great pain. He was in great pain and had many problems because he killed his Son Ganesha. He couldn’t forgive himself.
One important thing one must remember is that this at least didn’t happen on this world and in this time line; it happened in the world that the creation of Brahma’s fifth head. In the Hindu scriptures it says that Shiva cut off Brahma’s fifth head. Shiva did this by altering the universe, the time line and going back to the sources of creation its self. He altered the time line and the way the universe was. Shiva did this because he wanted to change the fact of him killing Ganesha and being condemned and punished by Brahma.
All Shiva needs to do is forgive himself and undo the changes he did to the timeline. He just needs to go through the process of forgiveness. For I an incarnation of Krishna forgive him. The Karma of what he has done has a way of being undone and will run out.
Brahma’s fifth head represents the original pristine pure version of ourselves and the universe before Shiva corrupted it to try to be the one absolute god. Yes Shiva wanted to overthrow all the other hindu gods and all other Gods and be thee one and only God. This is why in other life times I was Akenaten and I was the first one to start a monotheistic religion. I did it and did the Manjushri of what I did in starting monothesistic religions and dedicated it to the Universal One non individual self and dedicated it so Shiva would never be the absolute god over all that he wanted to be and dedicated it to undoing the harm that Shiva did.
Shiva has been forgiven and the Karma of what he did will one day run out. We will all reach the 5th head of Brahma and together we will all be one Universal One family.