well, consider that they never asked to be dropped into your tank, and that they only kill to feed themselves and remain alive (unlike human beings, who kill other animals and each other for just about any reason, including abstract concepts like religion and nationalism).
if they are in your reef, the best advise would be to try to get them out, especially if you don't want to see your invertebrate population, then your fish, start becoming food.
you can then either give them to someone who wants to keep them, or keep them in a species tank. they are very interesting critters, relatively "smart" and personable.
here's several galleries of pics showing how colorful and beautiful some are:
http://www.blueboard.com/mantis/pics.htm
note that i have kept smashers with various fish for long periods of time with no problems, so long as the mantis is kept well fed.