Socrates made a decision to die, and his reasons for this decision can be classified into three categories, according to their type.
Some of Socrates' reasonings on death deal with the intrinsic nature of death itself. He finds it “absurd if at my age I were disturbed at having to die,” because, more than stating categorically that death is not the “greatest harm,” it is for him a pleasant experience. He alludes to Homer's poetry, calling death “fertile Phthia” and seems also to be fatalistic: “be it so, if the gods will.”
The largest amount of discussion is spent on legal and moral reasons for submitting to death. To Socrates, escape would be unjust because it would be breaking the law to “repay injustice with injustice” and because it would break the life-long agreement with the state which has acted as his guardian, which agreement has been upheld in word and action, to abide by the decrees of the state. As creature to creator, Socractes is indebted to the state, and should honor it with obedience.
Most of Socrates reasons for remaining to die are entirely internal, dealing with his honor, consistency, etc.; the only hint of altruism is that he does agree, in passing, that he is “anxious about” the risks that Crito and others would run in arranging his escape.
This is a basic classification of the reasons which Socrates gave for his choice to die.
Frederick Copleston wrote an excellent book about Socrates (and other Greek philosophers). To buy his book, click here.