A single witness shall not rise up against a man on account of any iniquity or any sin which he has committed; on the evidence of two or three witnesses a matter shall be confirmed.
Deuteronomy 19:15  NASB

Then he [Jehoshaphat the king of Judah] charged them saying, “Thus you shall do in the fear of the Lord, faithfully and wholeheartedly. Whenever any dispute comes to you from your brethren who live in their cities, between blood and blood, between law and commandment, statutes and ordinances, you shall warn them so that they may not be guilty before the Lord, …”
2 Chronicles 19:9-10  NASB


The problem
Many seem to suppose that capital punishment was the rule in Old Israel. The contrary is true.


1. Only the Highest Court could give the sentence of capital punishment. This court (70 members) was thought to have begun with Moses (compare Exodus 18:22, Numbers 11:16).
2. The act of killing had to be proven (compare Deuteronomy 24:16).
3. The intention to kill (murder) had to be proven (Numbers 35:22-25, Deuteronomy 19:4).
4. Independence of the witnesses had to be proven during the interrogation (Exodus 23:1, Leviticus 19:16).
5. The intention of the witnesses to speak the truth had to be controlled during the interrogation in court (Deuteronomy 19:18-19).


Also in the period of the second temple (Mishnah)
6. In court (71 members, Sanh. 1.6) it was not permitted for anybody to give a speech to accuse someone of murder (4.1), only an interrogation of the witnesses was obliged and should be convincing (Deuteronomy 19:15). 
7. The decision of conviction was by a majority of at least two, a decision of acquittal was already by a majority of one (Mishnah Sanh. 4.1).
8. The trial was during the day and not at night (Mishnah Sanh. 4.1).
9. There were at least two clerks (Mishnah Sanh. 4.3).

It was difficult to meet all the requirements, so capital punishment was an exception in Jewish society, and an error was very rare (except in cases of lack of experience, intelligence or good will of the judges; compare 1 Kings 21:9-13).


Rabbinical discussion (about 80-140 AD)
Note the following quote, source: Judaism II, J.F. Moore, 1927, repr. 1970, Cambridge, Harvard University Press, p. 187.

"The Mishnah itself brands a court which executes one man in seven years as ruinous [note: destructive]. R. Eleazar ben Azariah said 'one in seventy years.' R. Tarfon and R. Akiba said, 'If we had been in the Sanhedrin, no man would ever have been put to death,' on which R. Simeon ben Gamaliel makes the obvious reflection, 'They would multiply murderers in Israel.' "


The tradition was that capital punishment was an exception in Israel.