why not think of it in terms of increasing the average amount of utility value of all sentient beings, with the caveat that it is also unethical to end the life of any currently existing sentient being.

If a consequential ethic has an obvious hole in it, that usually points to a more general divergence between the ethic and the implicit values that it's trying to approximate. Applying a deontological patch over the first examples you see won't fix the underlying flaw, it'll just force people to exploit it in stranger and more convoluted ways.

For example, if we defined utility as subjective pleasure, we might be tempted to introduce an exception for, say, opiate drugs. But maximizing utility under that constraint just implies wireheading in more subtle ways. You can't actually fix the problem without addressing other aspects of human values.