I'm not entirely sure that the aliens wouldn't announce their presence if they stumbled across us in their interstellar travels. If they're really that concerned about our well-being, wouldn't they tell us how to evade the dangers of global warming, nuclear holocaust, and overpopulation? Are they going to wait until the last minute to intervene on our behalf just before we wipe ourselves out? I think they would realize that an ounce of prevention is worth a pound of cure, and that the right thing to do is to come to our aid well before it's too late.

Anyhow, I wouldn't expect the aliens to be morally perfect angels. They may indeed have more advanced moralities than we do, just as we consider ourselves to have more sophisticated moralities than our slave-owning anti-democratic racist ancestors. But I bet they would still get in situations where they acted in their own self-interest rather than out of empathy for others. Perfection, I suspect, is an impossible achievement.