Remember the practical rules that will inevitably govern the behaviour of any alien "visitors":
1. They will be predators by descent at the apex of their food chain. 2. They will be utterly ruthless in their attitude to any competitors. 3. They will put their own welfare before ours in all cases. 4. They will assume that we operate as a species to the same rules.
Just put all this alien contact stuff through that filter and just hope to God that the UFOs aren't alien.
I'm not sure they will necessarily be ruthless. We're predators by nature because that is how life evolved and adapted on Earth, from the smallest prokaryote to Animalia. But for a species to truly evolve to an interstellar civilization, they must first survive themselves--i.e. not blow themselves to bits. We haven't overcome this and I'm often doubtful we will.
But any civilization that has, that has become interstellar, they may realize the true value of life and the sanctity of what species from neighboring worlds bring...
So I don't assume they would want to kill us. They /might/ want to kill us, but by virtue of them being able to traverse the black of space and visit other worlds, they overcame themselves one way or another and may actually be benevolent beings.
We're not dead yet, despite the historical writings indicating possible otherworldly visitors. No modern proof visible to the public has been shown, but what if they have? And if they have, why aren't we dead yet? If we aren't... perhaps they do value us. Or, maybe they don't value us as equals, but as potential chattel. Perhaps they've already modified our genetic structure, and modified us in ways to neutralize our threat potential, long before we ever knew what genes or tools were.
Or maybe they haven't.
Regardless, we're still here and any visitor that has seen us may have let us survive. If we were ever to encounter such beings publicly, I'd suggest we first not think like humans when addressing them. Or as an Earthling. So our natural reactions might be the wrong ones.