i have heard of this one, i personally think the fermi paradox + the great filter theory is closer to occams razor and a more succinct explanation to lack of direct contact. although if there is some sort of Necron like civilization out there we are pretty fucked
i don't know too much about fuel sources but i'm inclined to believe fusion or something we haven't discovered the use for yet would be more efficient/safer to use than waging war on organic life. if you had something like a dyson sphere the ship would provide all the fuel you could possibly need, at least for anything we can currently imagine
if aliens were to come to earth i think they would probably be looking for something they couldn't find elsewhere, something like a specific genetic sequence or dna, which might involve pasting all terrestrial life or might not
if you think of outer space as the size of all the oceans on earth, the amount we've explored or looked at in telescopes is the size of a teaspoon. the fact is we just don't know what's our there and it's probable that there are other intelligent species. if it can happen to us it can happen on other planets as well.