[MrAwesome]

Traditionally, the second one is viewed as terrifying because an alien race may see us as a potential future threat. In a lot of science fiction, they see some violent/awful news from Earth indicating that we're monsters, and decide that it would be better to go ahead and exterminate us before we can do unto others as we do unto ourselves.

So the thinking is less that we're a threat to them now (if they can get here, we certainly are not), but some intelligent beings may look at how we have treated one another over the last few centuries/millennia and decide that they don't want us (or any self-replicating machines we happen to plop out) roaming/colonizing/populating/enslaving/murdering/reeducating the galaxy.