So tell me, if I am happy by killing others, and I feel that their death is best for them, am I being moral according to your system?
Try thinking of it like this. In the moral equation your own happiness should carry no more and no less weight than the happiness of others. If you can conclude that as the result of the act that you are contemplating happiness for all those who could possibly be affected will see a net increase, then and only then can you consider the act moral. If killing infants makes you happy you must still consider the effect that your action will have not only on the infant, but the family of that infant, of the community in which the infant lives, and even the effect it will have on your family (what would your mother think?), friends and community. If after taking all this into account honestly and completely you can say that the result will be a net increase in happiness then you can consider the act moral.(BTW, I don't find joy in killing others). If you believe that the purpose of life to be happy, and killing makes a person happy, then on what basis can you (with your own standard of how life should be lived) judge another person?
Or do you presume to know what makes everyone happy and therefore feel that everyone should be bound to live by your opinion of what makes them happy?
But as Autodidact has already pointed out very few people actually take pleasure in the killing of other people. I think we have a natural aversion to harming other people. With the exception of the sociopath we all have compassion and empathy for other people. However this natural aversion however can be overcome when people give up their own sense of morality and succumb to the authority of another, even when that other authority is God.
Men never do evil so completely and cheerfully as when they do it from religious conviction. -
Blaise Pascal