How can you judge what is right or wrong? In the end, it all comes down to your subjective opinion.
With what evidence? The emprirical evidence which is before us; that pain which has been caused by you and the pleasure that you may have received.
Those that fostered the violence of the Inquisition were convinced, absolutely, that they gave mankind the greatest possible spiritual and eternal pleasure by torturing to death alleged "heretics", and wiping them out.
In ancient Rome, a family that gave birth to a baby girl often took that baby out into the wilderness and let it die of exposure. It was simple utility - girls cost much more money than they brought in, and families would be put in tremendous financial pain by allowing a girl to grow up and be married. Families were expected to provide a dowry when their daughter was to be married. The pleasure of a family outweighs the pain of one. That does not erase the horrendous nature of this deed.
Please understand, I am not saying that you would do any of these terrible things. I am simply pointing out where this reasoning can lead. You need an unchanging standard that is right at all times, in all cases, and in all eras. God is that unchanging standard.
You ask, "What if I don't believe in the Bible?" You can't have it both ways. You can't insist that a morality (in this case, the Bible) does not apply to you if you choose for it not to, while on the other hand insisting that you can measure what is ultimately good or evil for others in the world through utilitarianism. If someone else chooses to reject your premise and destroy someone else without regard to pain, you still really have no case to refute them, because in the end it comes down to your opinion.
I enjoy speaking (writing) with you. Be safe.
With regard to the Roman family, I would argue that their action may have made practical sense, but it does not necessarily increase happiness. The birth of the daughter may cause financial stress, but should we be solely concerned with hedonistic pleasures, such as those connected with pecuniary valuations? I think not, and when we see that emotional and intellectual pleasures and pains are more extensive and longerlasting than physical pleasures, one can argue that the family which does this has ultimately caused themselves greater pain by sacrificing their daughter. [Granted some utilitarians, such as, Peter Singer, place greater value on the physical pleasure, but as John Stuart Mill says, it is better to be Socrates dissatisfied than a fool satisfied, better to be a man dissatisfied than a pig satisfied. (I rather see Singer as that foolish pig.)]
People may missuse utilitarianism, just as they missuse all ethical theories, but that does not negate the validity of it. The sandard by which actions are judged is an objective one, since they are able to be measured, more so with the present advancement of science.
You need an unchanging standard that is right at all times, in all cases, and in all eras. God is that unchanging standard.
I completely agree that we need an unchanging standard, and I regard utilitarianism as that standard. It is my own opinion that our conception of God is more subjective than it is objective, because even if we acknowledge that there is objective evidence, through observation of Nature, for the existence of God, or a Creator, our knowledge of his character and commands is less objective, but is instead based upon the subjectivity of the men to whom God supposedly speaks.
The standard of utilitarianism, when appplied correctly, is analogous to a ruler or balance.
I enjoy speaking (writing) with you. Be safe.
Likewise.