Lord Vishnu is associated with Sattva Guna. Sattva Guna means purity and good.
- How sattva guna dominated Vishnu were able to Kill Rakshasa such as Ravana and Kamsa?
- Does Only Vishnu have Sattva Guna not his Avatars( Narasimha,Rama and Krishna)?
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"),
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.
Your presumption that Lord Vishnu is associated with sattva guna is incorrect. Sattva, Rajas and Tamas are Gunas of "NATURE", but God being the creator of nature is both a part of nature and at the same time different from nature. So to answer your question, the trinity, Brahma, Vishnu and Shiva are not bound by or characterized by any or all of these gunas but are free from all of these.
Lord Vishnu's Sattva guna is in accordance with his main aim, ie, Preservation. By Rajo guna, ie, characterized by desire, Lord Brahma creates. By Tamo guna, Lord Shiva destroys and by Sattva guna, the world is preserved.
Now when Lord Vishnu "kills" a demon, he is actually removing a potential threat to the existence of the world, and thereby preserving it. Also, when he kills the demons, Lord releases them immediate from all the sins and frees the soul. Thus, even when Lord Vishnu kills someone, He grants them liberation and moksha. This is how Lord Vishnu characterizes Sattva guna.
Whoever has been killed by Sri Vishnu was either cursed or had done penance in previous lives to be killed by incarnations of Sri Vishnu.No ordinary peole were killed by Sri Vishnu.Whoever is killed by Him get liberated immediately as lives of His incarnations show. So He does not kill out of anger :It is the grace arising out of Sattva Guna that liberates the souls.
In Srimadbhagavatam, we get : "Avataara hi asamkheyah hareh sattvanidher dvijaah" meaning therevare innumerable avataaras of Sri Hari, who is the abode of Sattva-guna and so all the Avataaras of Sri Hari must be of Sattva-Guna.His Avaataras make dharma-sansthaapana, which is a function of Sattva-guna as it preserves dharma.
You seem to misunderstand what the samskrut work 'sAtvika' means, like 99.5% of the populace. It DOES NOT MEAN 'always-peaceful', 'tranquil-always', 'calm-no-matter-what' frame of mind! Let me state for the record what it means. If you were fooled like majority others as to its meaning, here is a reference from a very popular and reputed samskrut dictionary source on what that word means, I have listed the first 6 meanings of that word (samskrut language has multiple meanings for a word depending on the context in which it is used):
'courageous' 'genuine' 'spirited' 'not artificial' 'virtuous internal' 'energetic'
As you can see, it does not mean what you were falsely led to believe it means (mostly Jainist & Buddhisht propaganda that was unfortunately revived in its death by the british and islamic miscreants or strange-bedfellows)! sAtvika mind denotes a very strong will/heart/spirit to do the right thing. Not a timid/serene/calm/peaceful mind at all! Sri Rama was 'sAtvik' as was Sri Krishna, and by above definition, it was precisely BECAUSE they were sAtvik that they were able to kill demons and evil people with ease! And they both ate meat per vAlmiki rAmAyana and vyAsa mahAbhArata.
My contention is that ONLY A physically strong person can be mentally strong as well. Hence meat eating, which provides significantly greater amounts of Iron and Zinc minerals and vitamin B12 and all amino acids, all of which boost testosterone in men significantly leading to strong physical strength, is a prerequisite for a sAtvik mind! A vegetarian man with lesser testosterone than a read-meat eating man is not stronger of the two, and such a weaker man cannot uphold dharma as well as a stronger meat eating man. Yudhistra was more sAtvik than anyone in mahAbharata, and he was a meat eater per vyAsa mahAbharata!