At its heart, the essence of murder is the sense that the victim's very existence is subject to his oppressor's discretion. A killer views all of life as his private domain, traveling across G-d's land as if were his own. For this, the earth must take revenge. Man has no right to destroy another. It cannot be.
Man can flatter the earth for only so long.
Ultimately, the earth must right itself, expressing the unity that defines its existence. True revenge rights a wrong and makes it whole. A persecuted man senses a deficiency in his very existence, hence the need to rectify the injustice. Revenge is sweet for it repairs the damage to one's self, restoring the pride and self-esteem of a healthy human being. 2 G-d remains forever whole, and nothing can truly sully His name.