1-Regarding your first question, please refer to the answer No.928 "The difference between Satan and carnal soul"
2-The purpose of man's creation is to achieve divine perfection. Allah (SWT) the Wise has granted humans all they need to achieve this perfection including carnal soul.
To explain more, Allah (SWT) has endowed mankind with three souls called carnal soul, self-reproaching soul and peaceful soul. In fact they are not three separate forces, rather they are three different attributes for human soul each called with a different name based on different modes of human soul; if the intellectual force of soul dominates other forces making them obedient[1], it will be called "peaceful soul" due to the tranquility and peace it earns in this state[2]and also because of the inclination toward what it selects as desired[3]. But if the intellect does not achieve this domination and fails in this struggle or battle[4] but a kind of repentance and remorse overtakes the soul then it is called "self-reproaching soul". And if the intellect surrenders without defending, it is called "carnal soul"[5]; because when the power of intellect loses its strength and obeys the satanic forces without defending itself, it is as if the soul itself has become the commander to the evil inviting man to indecent actions.
The way these forces gather in humans is like the gathering of an angel with a dog, a swine and Satan. If a fight breaks between them the one who defeats the others will rule them. Then any deeds by him or any of his attributes he manifests will suit this mode of his. Therefore human body is a gathering place for these four creatures; angel (the intellect), dog (anger)[6] swine (lust) and Satan (the estimative faculty).
A constant battle goes on among these forces in human soul until one of them dominates the other. If anger dominates, it commands human to tyranny, disturbance, animosity and grudge. If lust conquers, it will drive humans toward indecent acts and voracious eating and intercourse. And if satanic force triumphs, it will provoke the anger and lust adding up to what anger and animal attributes do. But the intellect is supposed to help the lust dominate the anger and break the force of lust through the force of anger and finally find and foil the tricks of Satan by its luminous sight.
Thus if the intellect dominates all other forces in such a way that these forces do not move an inch without the intellect's command, then the intellect will achieve its perfection; because all forces have reached balance and each will be fulfilled (due to lack of impediments). But if the intellect is unable to dominate them, they will dominate it using it in their own service. The dog that is constantly disturbs and bites or the swine that is committing vices or the Satan that ponders on how to dupe other forces utilizing their services and to satisfy the dog (the anger) and satiate the swine (the lust) therefore they will misuse the intellect as a powerful instrument to implement their purposes leading it astray.
To clarify the point, here is a simile; the gathering of these forces is similar to a rider who rides a horse and would like to head toward a destination. He has a dog and a spy of enemy accompanies him. The rider is the intellect and the horse is the lust, the dog the anger and the spy is that faculty of estimation which is in fact one of spies of Satan. Then if all of them come under the control and the domination of the rider, he will apply whatever is expedient to reach his destination. But if the horse or the dog takes the control, the rider will be drawn into hills and precipices and will die in pain. And if all of them come under control of the spy being duped by his tricks, the spy will lead them astray and have them entrapped by bandits.
There is an important point here in the obedience of these forces or powers from each other: if all forces come under the command of the intellect, they will reach a compromise and they will become like a single system; since there is only one active force that commands each one of other forces to perform its appropriate task. Therefore each force will act according to its purpose of creation complying with its due volume, time and quality. This way the soul and its forces will achieve their due perfection. But if the intellect is unable to dominate other forces, a fight will break and intensify until all forces are destroyed and the soul will be spoiled.
3-As mentioned earlier, carnal soul is a state of human soul where the force of intellect is overtaken by the anger and the lust. If the intellect dominates the anger and the lust then the human soul will not be called carnal soul rather it will be called peaceful soul. So carnal soul does not equal to the anger and lust, rather it is the state where these two forces dominate the intellect. Secondly, human's instinct and nature are not limited to his anger and lust whereas other forces such as kindness etc. are among human's instinctive or so-called innate forces which will be spoiled if carnal soul takes over. But if peaceful soul becomes dominant, then all these forces will flourish and reach perfection.
4- As mentioned earlier, the intellect is one of the forces of soul which is accompanied by lust, anger and faculty of estimation side by side. However, self-reproaching soul is a state of soul where the intellect has not become fully dominant and is still young. That is why following each failure, the soul becomes repentant[7].

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