I think my integrity as a person and the way I live my life would speak more volumes than to take part in dispensing a drug that would end a life.
And that's fine, you are perfectly within your right to not dispense the legally prescribed drug. However, wouldn't you make a stronger statement if you chose not to accept employment/compensation from a company that does dispense the offending drug? Wouldn't it do more for your integrity if you were not employed by a company that dispenses pharmaceuticals you find morally objectionable?