Comment by wostusername

Since everything in physics tells us that FTL travel is impossible (as far as I am aware), then when a civilization detects signs from another civilization in another solar system it knows that 1) that civilization was advanced enough to produce signals into space light years ago, and 2) by the time it reaches it at sublight speeds (hundreds, possibly thousands of years) it will be more advanced still. That means your extermination mission might be facing a technologically superior enemy with home field advantage.