Kali, means black, the symbol of tamas or darkness. She is the Goddess of evil, but engaged to destroy it; for the good cause as sometime evil can only kill the evil. Narrations in ‘Durga Saptashati’ states that Dewi Kali was called in for her ferocious forms to kill demons. She represents Tamas, ignorance and uncontrolled excitement and power, which is required to destroy the object. In bible too, Psalm 34:21 states “Evil will slay the wicked; the foes of the righteous will be condemned”.
She carries human heads and drinks blood because she killed numerous devils. In a narration, she killed Raktabeej (literal translation: Blood seeded), whose drops of blood falling on the ground created thousands of demons like him. So she killed him inside her mouth so that all his blood drops did not fall on the ground and thus the demon was killed forever.
Wears a garland of heads, reflecting various dark and destructive forms of evil and ignorance that lures people, sequentially. One errors compounds to another, one bad habits brings another and puts people to the worse. (e.g, drug addicts often become thief’s and sometime killers as well). An evil is born from another and leads to another. Garland of heads is the chain of evil.
Tamas or ignorance is uncontrollable. A story goes, that Devi kali was dancing seeped in tamas ignorant of anything else. She had to be woken up. Shiva, her husband, had to intervene to stop her dance by lying down. No sooner she stepped on him, she woke up from her trance and stopped dancing. This indicates that once the evil steers inside, people are unable to recognize it and are deluded in ways to follow evil. It stops only when a near and dear one is hurt.
She is shown with wild and primitive dress because evil emerges from wild and primitive thinking, which is darkness. Most evildoers never think much about cause and consequences, nor rise in spirituality. High degree of wilderness and selfishness is animal nature and tribal character.