Christianity teaches that
sin is that which goes against the
will of
God, and that He is primarily the injured party when one commits a
sinful act. This is an important distinction if you're trying to figure out what makes Christians tick.
Definitions of sin that do not take this into account tend to encompass acts of violence (physical, emotional, or spiritual) against other human beings and perhaps animals. Christians believe that we owe our existence to a loving God who wants us to take care of ourselves and live in harmony with Him, and so their definition also covers acts of violence against one's self and faithlessness toward God. Thus we see Christian opposition to suicide (assisted or otherwise) where a libertarian might say that the person has a right to do whatever he pleases with his life.