There are zillions of ancient societies, each with their own laws. What was legal in one society was not necessarily legal in another, in the same way that I can drive on the right side of the road in the United States but have to switch to the left side of the road in the United Kingdom. As such, strictly speaking the way you've framed this question there can be no answer, since you've extended the scope of investigation to all of humanity.

However, with Roman law it can at least be said that you're definitely off the mark. Seneca, and many imperial enemies, committed suicide, but they were not sentenced to do so by a court of law. Rather, Roman law did not recognize suicide as a legal offense, except (says the Digest) in the military, which had its own codes of behavior and discipline. Therefore, a suicide could not be punished as though he had been executed under law. Frequently those committed of capital crimes and executed had their property confiscated by the state upon death, thereby eliminating the ability of the condemned's family to retain any material well-being. Moreover, confiscation of property was humiliating, not only to the survivors but to the memory of the dead. While suicide to escape sentence was uncommon under the Republic, during which time flight into exile was vastly preferred, it became more prevalent under the Principate, when voluntary flight into exile was less of a real option. Enemies of the state (and, at the same time, of the emperor) frequently committed suicide lest their names and family suffer through the humiliation of trial, sentence, and loss of property and privileges. This, it must be said, fit rather neatly into some of the more philosophical positions on suicide. While late antique Christians and of course medieval commentators were strongly opposed to suicide, classical philosophers debated it and frequently condoned it. The Academics were generally opposed to suicide, but the Cynics and especially the Stoics approved of it. Notably, to the Stoics, control over one's own death was the only event over which one had total control--if faced with impossible circumstances, to the Stoics seizing control of life and death suggested virtue of the spirit. Thus, probably the two most famous Roman suicides, Cato the Younger and Seneca, were both Stoics faced either with certain death (in Seneca's case) or the humiliation of accepting an enemy's pardon (in Cato's). It should be noted, however, that though suicide offered an opportunity to escape the letter of the law, the manner and reason of suicide were still very much important. The Digest divides suicides up into honorable suicides and those done mala conscientia, from bad conscience. The former, often by sword (c.f. already in Sallust's writing the favorable description of Catiline's death charging into the midst of his enemies which, while technically not suicide, represents a similar attitude), incurred no infamia. The dishonorable suicide (and method mattered--hanging was, since Homer, generally considered a "womanly" death in the classical world) incurred infamia not only on himself but also on his widow and children, and was not allowed proper burial