Imam explains the matter through his theological works as following:
And if, God forbid, anger becomes permanent part of one’s nature, it will be more catastrophic; for the form that such a one shall acquire in the Barzakh and on the Day of Resurrection will be a beastly form, that too one which has no match in this world; for the brutality of the person in this state cannot be compared with any of the ferocious beasts. In the same way-as none of the creatures can touch this marvel of nature from the aspect of attainment of nobility and perfection, so also from the aspect of his capacity for degeneration and meanness and his leaning towards perverseness, man cannot be compared with any creature. It is about his perverseness that the Holy Quran says:
These are as the cattle-nay, they are worse in misguidance. (al-Quran)
It is about the hardness of the human heart that it says:
(Then the hearts of the Jews) became hardened like stones, or even yet harder. (al-Quran)