If Shiva is extremely soft, then he is also the toughest. Even the burden of entire universe may not slightly shake Lord Shiva, so he is called as Kathora. He needs this toughness at the time of punishing the rascals and at the time of final annihilation. The toughness of Shiva is assessed through Puranas only. Paarvati, the daughter of mountain, performed sacred austerities to attain him, but he discarded her in seconds and went away in seclusion again. Paarvati kept sobbing, but he never turned back. Due to such tough nature, he is never hesitant in breaking the illusive attachments of his devotees. Without this breakup, attainment of self-realization is not easy.
Shiva’s toughness leads a seeker towards strong determination. Sometimes Hathyoga is also helpful in the spiritual practices. Rigidness in resolute decision is not a bad quality. Sham(mind control) and Dam(control of sense-organs) which are classified within Shad-Sampatti (six-virtues), are required to solve this purpose only. Kathor is sometimes meant for cruel also. Shiva becomes very cruel in order to crush the ignorance. Apasmar, who is pinched down under the feet of Natrajmoorty, that demonstrates this toughness of the Lord Shiva. Bhaskar Roy presents a sweet description of this word ‘Kathora’—

घोरैका ते तनुरन्या शिवेति द्वे ते बिभ्रज्जगतां रक्षकोऽसि।
दुष्टा जीवा दमनेनैव रक्ष्याः पूर्णत्वाद्वा कथितस्त्वं कठोरः॥

Purport: You protect the world, sustaining two kinds of deities namely Ghor (fierce) and Aghor (mild). Mischievous entities can be protected through chastisement. You are called as Kathora because you are complete and fierce too. To cut down the hard knot of affection, is the true sense of worship of Kathora.