They find a Dyson Sphere (RAR!)

[K. A. Pital]

There is the problem of cheapness of interstellar warfare, though (throwing a fast rock against a Kardashev I planet is bound to decimate the civilization)...

Unless there are ethical constraints, of which we cannot be sure, nothing precludes a Kardashev II or III from wiping out unsuspecting tiny civs. It is also not certain that such an act could be traced back to them, if done right, so a police action for a stealthy galactic assassination is also questionable.

Of course, the opposite can also be true. Fear of retribution by a powerful overseer might keep belligerents from acting.

[Zixinus]

There is also the problem, in this case, that 800 years is quite a bit of time to crawl up on the Kardashev scale. By the time they get our messages, we might have the capability to send some nasty stuff their way, even if it is unlikely to cause extinction.

Hell, actually the existence of a K2 civilization should be a big incentive for us to climb that scale and spread out, even extrasolar. They might be K2 but we could still spread out and make us hard to kill. That would be a safe incentive for them or anyone else paying attention to not attack us, because a first strike might not wipe us out (like you could do to a species locked to one planet). No mass armament necessary, just spreading out. Sure, a K2 could still wipe us out but it would be harder. Meanwhile, we could actually start to communicate and maybe realize that the galaxy is big enough for both of us. With no FTL, any territorial dispute would take thousands of years to form.

[GrandMasterTerwynn]

My response would be "Meh." At 800 light-years distant, any message we could send to them would generate a reply in a minimum of 1600 years. That's quite a few years after I'm going to be dead. It will take many centuries (with the wildly optimistic assumption that we, as a species, won't get ourselves killed before then) to build up the industry and power-handling abilities needed to build something that could travel there. And then it'll be, at best, another thousand years after the ship launches before it arrives at the Dyson swarm, and another eight hundred years before we learn it got shot down by the local equivalent of a Patriot missile. All of which happens well after I'm dead.

Hence, my personal reaction would ultimately be "Pretty cool, but, meh."

Hell, actually the existence of a K2 civilization should be a big incentive for us to climb that scale and spread out, even extrasolar.

I'm not sure we're far-enough advanced as a species for the existence of a K2-level civilization to adequately motivate us to move upward. The discovery of such a civilization would be of immediate concern to no more than, perhaps, a few thousand people existing in the top 2-5% of the global economic strata. There are several hundred million humans currently alive whose dream is to achieve a standard of living enjoyed an American alive in 1917; and a couple billion whose standard of living hasn't progressed very far since about 1967. The discovery of a Dyson swarm does not illuminate a way out of our self-imposed resource and climate-change trap, nor does it teach us to not listen to our paleolithic urges towards selfish tribalism.

[The Romulan Republic]

There is the problem of cheapness of interstellar warfare, though (throwing a fast rock against a Kardashev I planet is bound to decimate the civilization)...

Relatively cheap, sure. But on the other hand, its money that they manifestly don't need to spend for any reason other than paranoid xenophobia.

Unless there are ethical constraints, of which we cannot be sure, nothing precludes a Kardashev II or III from wiping out unsuspecting tiny civs. It is also not certain that such an act could be traced back to them, if done right, so a police action for a stealthy galactic assassination is also questionable.

Psychology of an alien species at present is, of course, guess work at best, yes.

Of course, the opposite can also be true. Fear of retribution by a powerful overseer might keep belligerents from acting.

Yup. If there are other peer civilizations out there, observing, then you don't want to be known as the people who launch preemptive genocidal strikes, because that makes you a threat to everyone else.

There is also the possibility of domestic political constraints, but that goes back to the second point and is basically a big question mark.

[K. A. Pital]

There could be other reasons to attack once you are on the galactic scale. As you said, alien psychology (do they have one?) is guesswork. It could be incomprehensible intelligence of a non-biological nature, for all we know. Antropomorphism and the tendency to project is not helping us with things like alien contact.

[The Romulan Republic]

Perhaps.

Its simply a question with too many unknowns to do much more than speculate.

Still, its hard to see a pragmatic reason for making a policy out of preemptive strikes on far weaker civilizations. Any reason for doing so would likely be cultural/political, not one based in practical need.