Two wrongs make a right

From Wikinfo

Two wrongs make a right is a commonly used abbreviation for the belief that, if one wrong is committed, a second wrong will cancel it out (such as capital punishment for murderers, i.e. killing the killers). More common, however, is the reverse expression "two wrongs don't make a right" (used, for example, by opponents of capital punishment).

References

• Adapted from the Wikipedia article, "Two_wrongs_make_a_right" http://en.wikipedia.org/wiki/Two_wrongs_make_a_right, used under the GNU Free Documentation License