Sergei Kravinoff was born into the Russian aristocracy before the Revolution of 1917. His parents fled with him to America, most likely when he was a teenager, where they found a home in Upstate New York. The Kravinoffs must have found a way to bring much of their wealth with them to the New World, as the mansion they lived in was quite lavish. They also had a large amount of land, in which young Sergei practiced his skills as a hunter, carrying on the aristocratic tradition.
Despite escaping Russia with their lives and their wealth, the Kravinoffs found life in America to be dull. Sergei's family life was likely unpleasant, as his mother could fly into sudden rages and eventually committed suicide in a psychiatric hospital. (The Sensational Spider-Man Annual 1996) (She once slapped a maid after she stopped Sergei's mother from stripping down and jumping into a fountain in France. (Web of Spider-Man vol 2, #7)) His father was also a philanderer, who had a child (Dmitri Smerdyakov) out of wedlock with a servant girl. Sergei eventually left and became world famous as a big game hunter. A newspaper journalist, who couldn't spell the name Kravinoff, dubbed Sergei "Kraven the Hunter."
While on safari in Africa, Sergei met a witch doctor that gave him knowledge of jungle elixirs and potions that increased his strength, speed and agility. Using these natural chemicals made Sergei the greatest hunter of all time. After capturing every kind of dangerous animal known to man with his bare hands, Kraven became bored. He took up the challenge presented by the Chameleon to hunt Spider-Man, who had defeated him months beforehand. J. Jonah Jameson called Kraven, "...a living legend! The greatest hunter of all time! He's trapped and beaten every kind of beast that lives--and always single handed!" (Amazing Spider-Man #15) Loss after loss to Spider-Man caused Kraven to see losing to his enemy as a threat to his honor.
After years of humiliation, Kraven finally captured Spider-Man, drugged him, buried him alive in a grave and took his identity, to prove that he was superior to "the Spider." He took to crime fighting, killing the petty thugs he came across. After Spider-Man awoke from his drugged state and dug himself out of the grave, he went after Kraven with a vengeance. Kraven loosed the cannibal, Vermin, into the city, forcing Spider-Man to go after him. After Spider-Man left, Sergei committed suicide. (Amazing Spider-Man #294)
Years later, Sergei's wife, Sasha, resurrected her husband via a dark magic ritual. This infuriated him, and he killed her. The only way that he can die again is if its by Spider-Man's hands, but Spider-Man is unable to do so, for various mystical reasons. Kraven is now back to the hunt, needing to die from an enemy who refuses to kill him. (Amazing Spider-Man #637)
Genetic illness and Jungle Potions
Its unclear the nature of Kraven's mother's illness, except that she could be normal for months on end and then suddenly explode. Nor do we know anything about the jungle substances that Sergei used before a hunt. They were herbs, roots, juices, poisons, fruit and flowers. He told himself to "Let them pervade your mind...widen it...shatter your Kravenness...and let in Spiderness." Soon after drinking the concoction, Kraven hallucinates a giant spider, made up of thousands of tiny spiders, coming to kill him. Though he struggled, he hallucinated that the spider consumed him. (Amazing Spider-Man #293)
Kraven the Hunter: Alpha Male
Spider-Man straight up calls Kraven "the macho type," right before he gets shot with a tranquilizer dart. (Web of Spider-Man #31) Testosterone is the primary male sex hormone and has been found to be linked to increased aggression and sexual behavior in both men and women. Kraven was never short of women in his life: his wife (Sasha), his lover (Calypso Ezili), the mother of his illegitimate son, Alyosha Kravinoff, a Russian baroness that he attempted to have his way with and Gwen Stacy.
A study from 1990 done with male prisoners found that inmates with higher than average levels of testosterone were more likely to commit violent crimes, have more sexual partners, have more prison violations and more likely to be delinquent. Sergei was always the tenacious type. Even after being deported from the U.S. back to Russia after his first time in prison, he found a way back to New York.
Kraven is obsessed with his own death. Before beginning his Last Hunt, he contemplated his life. "I have found morality, I have found meaning--in the hunt. But I cannot escape Time forever. Herbs and roots cannot rejuvenate a dying spirit--or heal a heart crushed by the weight of a corrupted Age. I will die soon. I must die soon. But not yet." (Web of Spider-Man #31) After being returned to life, he was no more happy. He begs Spider-Man to kill him and accuses Sasha, "It was perfect. It was a masterpiece. You took that masterpiece away from me." (Amazing Spider-Man #637)
Mood swings and thoughts of suicide was often symptoms of depression. Sergei's mother had such mood swings and may have passed the trait on to her son. Depression can be a genetic disorder, and the substance abuse that Kravinoff engaged in, plus the numerous defeats at Spider-Man's hands made his suicide much more likely. Add the fact that he had numerous firearms and weapons in his mansion and his death became very certain.
Kravinoff felt that he no longer belonged to this world. He was a relic of an old way of life. All he had was "the hunt" and he had conquered every animal known to man, leaving him without a challenge. Defeat by Spider-Man gave him a new challenge, one that he became obsessed with. For a man as proud as Kraven the Hunter, not being able to best a man in a costume grated at him. Add to that the genetic background of depression and recurrent hallucinogenic substance abuse and Sergei's psyche became very unstable. His mind is no less fragile after being returned to life, as he is willing to kill others to goad Spider-Man into returning him to the grave.