"would you not agree that murder and rape are objectively wrong rather than a matter of opinion"
No I would not.
Things depend very strongly on how you are defining those things, for a start. For example, in the UK, it was not considered rape to, well, rape, if the person you raped was your wife, up until distressingly recently. Every day there are people whining on about "grey areas". Then you have things like execution. Is that murder? Or not? Depends on your perspective. Is it murder if you are provoked enormously? Self defence?
And that's just the first 2 thoughts - I mean there's loads.
I mean come off it. You must be able to think of these things for yourself!
Of course morals aren't objective.
Is this all to detract from the "I've balanced out my tally in my head and I come out AOK" thing?