a) Any alien civilization advanced enough to not only locate us, but more importantly to travel to us within a meaningful timespan, would be immensely beyond us in terms of technological (or whatever is analogous to technology ...maybe instead of 1s and 0s it is biological in nature) advancement.
b) That means that if that alien lifeform was to be hostile there is precious little we could do to stop them. Looking at human history, every single time an advanced civilization met a less advanced one, the less advanced one paid in some way or form. Why should it be different on a galactical scale?
c) This does not even necessarily have to mean that the alien lifeform is malevolent. Maybe they only mean to 'help us better ourselves' and deem that, since we are not as 'enlightened as they are,' it may be most prudent to 'expedite' things. Or maybe they come along with some pathogen with which we have no resistance. Or maybe some accident occurs (e.g. their craft, or what we conceive as their craft, is fired upon by some overzealous air national guard chap, triggering a response from them for which we have no answer). Or maybe their presence so disrupts society (imagine what the PROVEN existence of aliens would do to some socio-religious nodes. Many would quickly include aliens in the greater works of God, but some would see them as demons and go loco ...maybe even attacking them). Think about market reaction ....
Anyways, right now we may just have captured the attention of some malevolent entity that is alien to us, that is heading our way. We expect humanoid lifeforms, but I doubt any aliens would be humanoid (this is not star trek). Furthermore, for them to travel immense distances means they either have extremely advanced technology (i.e. maybe using wormholes), or else they have 'technology' unlike anything we have even conceptualized (maybe they are not corporeal, but can instead project their wills and intent).
Or maybe there is nothing out there but empty space and infinite darkness (which I doubt from a mathematical point of view).
But even if that were the case, broadcasting would be like a person sitting on a log in the middle of some nameless vast ocean, and tapping hard into the water with a stick trying to see if he can attract something to the surface.
There is a chance nothing will come.
Or maybe some glittering fish with omni-colored scales.
Or maybe the last thing he will see is some dark cavernous maw rising up from the depths, ringed with halos of serrated teeth and retching like the very belly of hell!
Hey, as a Kenyan there is a story I was told when I was a child about a famous medicine man from our culture who allegedly prophesied about a race of men who wear clothes of many colors that made them look like butterflies, and who would come riding on an iron snake. That people would welcome them, and that for a time they would suffer greatly for giving that welcome.
Maybe right now some iron snake steams across the cosmos, and rather than seeming like flittering butterflies the riders of that train are veritable devils and monsters of yore!