Judaism places much stress on performing mitzvot, carrying out God's commandments. However, a "mitzvah haba'ah b'aveirah" - a mitzvah based on an aveirah (sin or "illegitimate means") - is forbidden and is not considered a mitzvah. For example, if one uses a stolen lulav and esrog on Sukkot, it is not a proper mitzvah. Similarly, if money is stolen, it cannot be used to give tzedakah (charity). In fact, the sages indicate that it is better not to do the mitzvah at all than to do a mitzvah haba'ah b'aveirah.
Eating meat is arguably a mitzvah haba'ah b'aveirah, actually ...
