If they start exterminating or enslaving us I think it would be very relevant to the question of whether God exist or not
why didn't God tell us if he knew? That would be an important piece of information to hold back on
OK, here's an example of something that would shake my belief in God.
Let's say that tomorrow, the world is perfected. All wars cease, people universally decide to be nice to each other. Food production is perfected. All illnesses are cured. Each and every person is able to discern precisely what their individual nutritional needs are, and they are able to perfectly supply those needs...
Perfection. The entire world's ecology, economy, politics, beliefs are all in balance with no suffering. No one and nothing is excluded from this.
with me so far?
Then... aliens come and zap the whole planet nothing survives; there is only suffering and then death.
OK... that would be a bit of a shake-up for my belief in God. But until there is some sort of balance, something approaching perfection, I think a cosmic do-ever is possible and it doesn't refute the existence of God. Is God benevolent in this circumstance? Not so much. But I'm still not certain about that. I hope that God is benevolent. But I certainly don't have proof of that. That's why an extreme hypothetical that includes tremendous never-ending suffering doesn't effect my belief in God.
I do have a lot of confidence in people, though. And I think that is affecting my answer somewhat.
Your example was: "Aliens came and enslaved us all." As long as people are still alive, I believe there would be a chance to throw off the shackles of enslavement. That confidence... yes, that would be shaken in the circumstance you described. But it doesn't affect my belief in God.