It takes intellectual courage to kick yourself out of your emotional incredulity and persuade yourself that there is no other rational choice. - Richard Dawkins

I can think of no moral objection to eating human road kills except for the ones that you mentioned like 'what would the relatives think about it?' and 'would the person themselves have wanted it to happen?', but I do worry a bit about slippery slopes; possibly a little bit more than you do. There are barriers that we have set up in our minds and certainly the barrier between Homo sapiens and any other species is an artificial barrier in the sense that its a kind of 'accident' that the evolutionary intermediates happen to be extinct. Never the less it exists and natural barriers that are there can be useful for preventing slippery slopes and therefore I think I can see an objection to breaching such a barrier because you are then in a weaker position to stop people going further. Another example might be suppose you take the argument in favour of abortion up until the baby was one year old, if a baby was one year old and turned out to have some horrible incurable disease that meant it was going to die in agony in later life, what about infanticide? Strictly morally I can see no objection to that at all, I would be in favour of infanticide but I think i would worry about/I think I would wish at least to give consideration to the person who says 'where does it end?'

It is interesting to wonder whether taxonomists of the future may regret the way our generation messed around with genomes.

Science is the best way to do anything (if you want to do terrible things with technology, a terrible weapons for example science is the best way to do it because science is the best way to do anything)