OK, I realise this doesn't exactly constitute a philosophical proof.

I've been mulling over your question about what philosophy my Dad's starter explanation of democratic freedom led to. For reasons that go beyond point-scoring pedantry, the question seems clearly to require a working definition of philosophy itself or of 'a philosophy', at least. Break it down to its Greek roots to 'love of knowledge' or 'love of wisdom' and you've got something that seems clearly weighted to epistemology. This is limited, obviously, but this is where I feel your inquiry is leading, to whit, yes we don't really like the idea of murder, we wouldn't want anyone to do it to us or our loved ones, even to strangers, but how do we
know it's wrong, absolutely, in such a way as allows us to legislate against it? To put it in, I think, Kantian terms, we think we know this, but how? How is this knowledge given to me? Note that Kant was a Christian, but never felt able to resort to God as an explanation. From his philosophical perspective, it isn't one because it simply removes the question to a transcendent, inexplicable realm, an unknowable realm. God is one of Kant's noumena - an unknowable, something about which we cannot have any certain knowledge. Hence, it has nothing to do with
philo-sophy.