Metaphorically Krishna represents soul atman existing in human body. It is an absolute reality of life. Souls’ atmans exist and so do we. The existence of humans is directly based on existence of a spirit, a consciousness in their heart.
As detailed in Bhagavad Gita we are primarily a spiritual being manifesting human form. We are a Krishna that needs complete removal of dross impurities within. Indulging in spirituality we could cleanse self of dross impurities, realize our Krishna forever.
In real life also Krishna is considered an avatar, a messiah savior the advent of whom gets necessitated when society reaches a point of no return. In times of extreme distress, strife advents an avatar to cleanse the society of its ills, re-establish Dharma (righteousness).
Compared to Krishna Lord Shiva is only an attribute of Brahman (God Almighty) having nothing to do with absolute reality! A person by the name of Shiva never existed in this world.
As mankind, masses were unable to conceptualize Brahman in all its glory; they created many attributes of God Almighty. Hinduism is full of gods and goddesses.
Lord Shiva in Hinduism represents an enlightened one. Can we say Mahavira, Gautama Buddha, Jesus Christ and Prophet Mohammed reached state of Shiva, yes spirituality says so. In recent times even Sri Ramakrishna Paramhansa and Maharshi Ramana became a Shiva.
The coiled serpents around neck of Shiva represent a fully awakened kundalini shakti, the coiled Serpentine energy lying dormant in base of spine. The blue body of Shiva represents the enlightened one retaining the Vish (poison) in the body. The Amrita (elixir of life) was used for full awakening of kundalini shakti and the Vish (poison) retained in neck or body.
By contemplating on Samudra Manthan story, we can realize how Amrita and Vish get produced. Every single moment thousands of positive and negative thoughts clashed in our thought process. This resulted in a battle of Mahabharata, a war of gigantic proportions called Samudra Manthan. This battle yielded both Amrita and Vish. For becoming an enlightened one Amrita was necessitated, and Vish (poison) retained in the body.
The ultimate essence of Bhagavad Gita lay in the fact that human form is the highest manifest stage in cosmic life cycle. It is only in human form the Krishna within gains release from cycle of birth and death forever. As a human being we are duty-bound to help Krishna within. By indulging in spirituality as detailed in Bhagavad Gita we can finally reach the summit, becoming an enlightened one, a liberated soul atman forever.
Summarizing it all, the cosmic journey of life is travelled by Krishna within that desires negation of dross impurities within forever. When human beings indulging in spirituality reached state of Shiva, the liberated one… the Krishna within finally regains its lost original pure prime pristine primordial form. As pure soul atman it need not manifest a human body again. The need for human body arose as long as single trace of dross impurities remained within Krishna.
With human beings reaching god the circle of life finally completes and one realizes the true inner self, Krishna within.
As a human being we should not confuse between the states of Krishna and Shiva. Krishna is a liberated soul atman and Shiva the state of an enlightened one. Human beings first become a Shiva and then reach the state of Krishna, the liberated soul atman.