Krishna is reincarnation of Vishnu. He is very popular God because of his childhood frolics with Gopis (the wives of cowherds) and miracles. This facet of His life is full of romance.
He is shown carrying a flute and herding the cows because he belonged to the Yadav caste which reared the cows. When he played the flute, a magic spell was created and animals, birds and moreover Gopis became mesmerized and felt a force which carried them towards him. He flirted with numerous Gopis but his favorite was Radha.
Afterwards, there seems to be a total change in his personality. He became a very serious characters who sided with the righteous and helped them decimate the ignoble.
He became the director of war called Mahabharata between the cousins and preached a sermon to hesitant Arjuna about what is wrong and what is right and it was the pious duty of a righteous person to destroy the evil.
He was born in Mathura in the Yadava tribe. His father was Vasudeva and mother was Devaki. His birth was also very dramatic. Devaki, his mother, was the sister of ruling king Kamsa who was a tyrannic ruler. It was prophesied that eighth child of Devaki shall kill Kamsa.
For this reason, Kamsa put Vasudeva and Devaki in a dungeon and killed all his 7 children. But when Krishna was born and since it was all Maya, the sentries fell asleep, locks of the dungeon broke automatically and a baby girl born to a friend of Vasudeva called Nanada was exchanged and was duly killed by Kamsa who thought that problem is now solved as he had killed all the children of Devaki.
As he grew up, Kamsa traced him and made many attempts on his life and he slew the Kamsa and became the king of Mathura. But even now he did not have any peace because he was pressed between two hostile kingdoms namely that of brother-in-law of Kamsa Jarasandha and unnamed Yadav king.
He left Mathura and with his followers travelled to a place called Dwarka in the Saurashtra presently a part of Gujarat. He married Rukmini, daughter of king of Vidarbha his chief queen. Some say that he amassed 16000 wives and 180000 sons. This is only a mythological story because only a man with supernatural powers can perform such a mind boggling feat. Since he was the reincarnation of God, it was not impossible for him.
After the end of Mahabharata in which the wicked Kauravas perished and Pandavas become victorious, he settled them on the throne, he returned to Dwaraka.
Here ominous portents beset his regime as his clan members drank heavily and indulged in quarrels. He banned the liquor but it could not stop the feud. His son Pradyumna was killed before his eyes. His bother Balarama also died.
Dejected Krishna wandered in the forest and was hit by an arrow in the heel by a hunter as the hunter mistook him for a deer. He had a weak heel like Achilles it was a vulnerable spot. He died and city was swallowed by the sea.
Such is the story of Krishna. Even Gods perhaps with the exception of Siva have to die like other mortals. Also when God descends to Earth from heavens then he has to be like other people although adorned with extra powers.