> Why send a group or probe lightyears away just to kill or enslave a primitive species?

That question makes no sense. It presumes that they are at all interested in, or even notice, us. We're special, from our PoV. That doesn't mean we are special.

Did you notice how many ants you drove over on your last car trip? Did you even care?

To a space-faring species, we are probably just as interesting as ants. After all, you are assuming that they'll send their brightest minds to visit us, but it is equally likely that we are "discovered" by their equivalent of joy-riding teens.