That is (most likely) true, and is not something I deny. What I
do doubt is that all sentient alien races will have compassion, or at least compassion as we humans like to call it. We know only what life on Earth looks like, and how it behaves, but that is not a valid indicator on how life elsewhere must be.
But that's not the part I'm objecting to (and yes, I'll take issue with whatever part of your argument I want). Obviously if we encounter a civilisation that tries to destroy us, we are allowed to act out of self-defense. And perhaps there are even instances where we would be forced to perform a pre-emptive strike.
However, that kind of situation (which is what you're now arguing for) is something quite different from the "If they have resources we want/need, we get in there, and if they don't like it, we nuke them from orbit."
That's something else entirely. Now do you defend that, yes or no?