How Sattva Guna Lord Vishnu killed Demons or evil people in his incarnations?
Lord Vishnu is associated with Sattva Guna...
Relating 3 modes of nature with 3 Gods is insulting. Where BrahmA and Shiva are associated with Rajas and Tamas respectively. BrahmA is not only a homonym with Brahman but also quite near and also he is the creator of the universe. Shiva/Sadashiva/Rudra/MahAdev is equally respected among devotees. Now someone (may be a Vishnu worshiper) has ignorantly associated Triguna to Trideva and secured Sattva for Vishnu. Good that they haven't associated GyAna, Karma, Bhakti with Trideva or Triguna.
By the way, God's unmanifested nature which is the eternal truth, is transcendent to all 3 modes of nature.
Among 3 modes, domination of Sattva is the manifestation of the God's own manifested nature. If you want to understand the adverse effects of Rajas (translated as "mode of passion") and Tamas (translated as "mode of ignorance/darkness") then refer BG chapter 14. Then think, if we should even dare to relate Gods with the modes of nature.
After understanding those characterstics, one may read BG Chapter 16 on Daivi SampadA and Asuri SampadA. The Sattva relates to Daivi, while other 2 towards Asuri.
How sattva guna dominated Vishnu were able to Kill Rakshasa such as Ravana and Kamsa?
Because Sattva dominated, Vishnu was able to kill demons. Here is how.
Killing is just an event, like sleeping, playing, eating etc. But Sattva (or Rajas, Tamas) here is a state of "consciousness". Every action can be divided into these 3 modes based on domination.
When a person acts (here "kills"),
- For sake of duty, then that is Sattvik
- For any kind of desire, then that is Rajasic
- For illusionary outburst, then that is Tamasic
The demons or demonic nature is created due to extreme influence of Rajo Guna. So actually when Vishnu killed RAvana, Shishupala, he reduced the dominance of Rajas.
Even for common people Sattvik killing (i.e. done without false ego or desires) doesn't result in sin or attachment.
Does Only Vishnu have Sattva Guna not his Avatars (Narasimha, Rama and Krishna)?
I don't see any reason why the Sattva is not dominated among the Avatars of Vishnu/Shiva. Everyone incarnated and performed their Swa-Dharma, which is the nature of a SAttvik person.
 BG 8.20 — Yet there is another unmanifest nature, which is eternal and is transcendental to this manifested and unmanifested matter. It is supreme and is never annihilated. When all in this world is annihilated, that part remains.
 BG 18.23, 18.24, 18.25 — That action which is regulated and which is performed without attachment, without love or hatred, and without desire for fruitive results is said to be in the mode of goodness.
— But action performed by one seeking to gratify desires, and enacted from a sense of false ego, is called action in the mode of passion.
— That action performed in illusion and without concern for future bondage or for violence or distress caused to others is said to be in the mode of ignorance.
 BG 17.4 — Men in the mode of goodness worship the demigods; those in the mode of passion worship the demons; and those in the mode of ignorance worship ghosts and spirits.
 BG 18.17 — One who is not motivated by false ego, whose intelligence is not entangled, though he kills men in this world, does not kill. Nor is he bound by his actions.
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