Evolutionary speaking aliens would be clever to destroy us as soon as they find us, it reduces competition for resources, it eliminates possibility of future threat.
First of all, evolution has absolutely nothing to do with that so you shouldn't have started that sentence with "evolutionarily" (or, in your case, "evolutionary").
Second, the same argument could be applied to conclude that every single nation on Earth should try to destroy every other nation on Earth - it reduces competition for resources, it eliminates possibility of future threat.
In our history there is not one single example of a lesser advanced society meeting a more developed one and that society not being completely destroyed in three generations.
A http://en.wikipedia.org/wiki/Indigenous_peoples_of_the_Americas of http://en.wikipedia.org/wiki/Bushmen http://en.wikipedia.org/wiki/Australian_Aborigines.
-Any FTL drive usage is visible througout the galaxy.
Oh really? You clearly must know quite a bit more about FTL drives than I do, but you have to give me a break - no human has ever even seen one so far so I had nothing which to study about them from. But, pray tell us more of what you know about them. I, for one, am most curious.