There are many misconceptions that Lord Shiva is Tamasic God and scriptures of Him are Tamasic Scriptures. Some people even suggest not to read His stories as they are ostensibly believed to be tamasic puranas. This concept of Tamasic Puranas can be refuted.
Shiva Purana clarifies this by saying Rudra as Taamasic outside but Satvic inside.
परं ब्रह्माद्वयो नित्योऽनन्तः पूर्णो निरञ्जनः।
अन्तस्तमो वहिःसत्त्वस्त्रिजगत्पालको हरिः।। ५९॥
अन्तः सत्त्वस्तमोबाह्यस्त्रिजगल्लयकृद्धरः।। ६० ।।
I am the Supreme Brahman the eternal one, endless, perfect, complete and spotless. Visnu, the protector of the three worlds, has tamas inside, but Sattva outside. Hara who causes dissolution of three worlds has sattva within, but tamas outside.
एवं गुणास्त्रिदेवेषु गुणभिन्नः शिवः स्मृतः।। ६१।।
Brahmā on the other hand having rajoguna inside as well outside, creates the three worlds. Thus all the three gods have their respective qualities while Siva is different from all the gunas.
But those people say this could be an interpolation. Indeed there are many contradictory statements in Smriti (in all Puranas). Words of Shruti are considered as ultimate Truths. When Smriti contradict each other, people resort to Sruti to know the truth.
So, do Vedas or Shruti describe Shiva or Rudra with gunas other than Tamas?