When Socrates was in prison, he had a choice whether or not to drink the hemlock poison. It would not be wrong for him to do this because it was not a crime against anyone. David Hume, in "On Suicide" says, "If suicide be criminal, it must be a transgression of our duty to either God, our neighbor, or ourselves." Socrates drinking the hemlock would not be a transgression against his "neighbors" because many people wanted Socrates to die. It was not a transgression against himself because Socrates was not afraid of death. It was not a transgression against God, because according to Meletus, Socrates did not believe in any gods.
In Plato"s "Apology," when Meletus accuses Socrates of not believing in any gods, Socrates proves him wrong by stating that he does believe in divine beings, and teaches about them. He even was said to have a daemon, or spirit guide, that he believed to be guiding him. Therefore, Socrates believed in gods. By committing suicide, Socrates would be committing a transgression against God and against the laws of nature.
When Socrates was put on trial in Athens for corrupting the minds of the youth, he was given a death sentence. Socrates was going to die anyway. Killing himself would not violate the laws of nature because Socrates was supposed to die due to the consequences of his trial.
By drinking the hemlock and taking his own life, Socrates would not be letting himself die of natural causes. Socrates, himself, believed that death is supposed to be an unplanned event. By drinking the hemlock poison, Socrates would be going against his own teaching and beliefs by not letting himself die of natural causes. He would be disturbing the course of nature and the natural flow things.
In David Hume"s "On Suicide," he says, "It is impious, says the modern European superstition, to put a period to our own life, and thereby rebel against our creator: and why not impious, say I, to build houses, cultivate the ground, or sail upon the ocean?" This means that taking one"s life is no worse than doing things such as building houses, and any other things that disturb nature that humans do every day. In drinking the poison, Socrates would not be making any more of a disturbance to nature than the carpenters do when they are building a house. It was Socrates" fate to die, so it wouldn"t be wrong for him to drink poison, which would make him die anyway. He would just be fulfilling the fate that was given to him.
You say that Socrates would be fulfilling his fate to die by drinking the poison. However, Socrates" teachings were somewhat in opposition to the notion of fate and its controlling influence on a person's life. He believed humans were responsible for their own destiny. Socrates would have his students reflect on the mistakes and bad choices that they made, and learn to live differently and move forward. By drinking the hemlock and ending his life after being put on trial, Socrates would be going against what he believed and taught his students.