Bhagwan Shiva is Nirguna actually. He is neither Tamasic or Sattvik, he is Trigunatita (beyond the three gunas).
Yes, for keeping the world stable, he himself takes the role of destroyer in the form of Rudra and eliminates the entities that cause instability. In this process, he assumes Tamo Guni attire, wearing garland of skulls, Halahal poison, living in cremation grounds with ghosts, spirits etc.
But the same Mahadeva is also in the Satvik Yogic state as he carries Maa Ganga, the embodiment of purity in his matted hair and Chandra Deva, the moon on his forehead.
He also does the Rajasik activities of creation, he shows his art in the form of Nataraja, the Ultimate Dancer.
So he takes up all 3 gunas and yet is beyond all three gunas.
Har Har Mahadev 🙏