Lone Witness

לא יומת על פי עד אחד    

The Rambam in the beginning of Ch. 6 in the Laws of Murders discusses those that go to the arey miklat and those who don't.  He says one who kills in a manner which involves negligence does not have the atonement of the arey miklat and will have to be a fugitive from the goal hadam for his entire life.  He adds in Law 5, וְכֵן כָּל הָרַצְחָנִים שֶׁהָרְגוּ בְּעֵד אֶחָד אוֹ בְּלֹא הַתְרָאָה וְכַיּוֹצֵא בָּהֶן אִם הֲרָגָן גּוֹאֵל הַדָּם אֵין לָהֶם דָּמִים. לֹא יִהְיוּ אֵלּוּ חֲמוּרִים מֵהַהוֹרֵג בְּלֹא כַּוָּנָה.  The Raavad asks תימה הוא ואיך נאמין בעד אחד  להתיר דמו של זה לגואל הדם.  How can one trust one witness to kill the supposed killer?  It is clear from the Rambam that there is no heter for Beit Din to act based upon one aid to have someone killed but that does not mean one witness can't be believed.  Although one witness can be believed, he does not give the Beit Din the power to carry out capital punishment.  However, outside the context of a proper judicial act, one may take into account the testimony of one witness.  Hence, the goal hadam may act based upon one witness.