Steven Hawking recently suggested that a meeting between humans and an extraterrestrial species might not turn out well for the humans. As an example he used the encounter between the native inhabitants of the Americas and the invading Europeans. It went badly for the natives. The newcomers had an overwhelming advantage in terms of weapons, and in all of the direct confrontations it was really no contest. There were some successes, like the Little Bighorn, but the eventual result was never in doubt.
It is certainly possible that a space faring alien race that came to Earth might possess weapons so superior to ours that a violent confrontation would inevitably result in the defeat of humanity. And it is possible that the alien species would be as aggressive and violent as we are.
But consider this. Because there are something like 40 sextillion star systems (that is a four followed by 22 zeroes) it seems likely that life has arisen in a very large number of places. In some places this process is undoubtedly in its early stages, and only very simple, unicellular organisms exist. Some, however, almost certainly began the process of evolution long before it started here. A species that became intelligent and developed technology on one of those planets would have had time to progress in scientific and engineering knowledge far beyond what we have managed so far.
For an aggressive and violent species (such as humans) improvements in science and technology are accompanied by improvements in weapons. Sometimes the need to create new weapons and improve existing ones is actually the driving force behind new discoveries. We progressed (it is progress, right?) from weapons used in hand-to-hand, face-to-face, one-on-one combat to our current ability to direct a strike by an unmanned Predator in Afghanistan from a command center in Las Vegas. This makes inflicting death and destruction more impersonal. It is easier to push the button that destroys an enemy thousands of miles away than to hack him with a sword when he is right in front of you. The process has been sanitized and is generally less traumatic for those who fire the weapons.
Not only do we have weapons that can be activated from very far away, our arsenal contains some items that are so devastating that we decline to use them, at least for now. No nuclear weapon has been used since 1945. We are constantly reminded of the devastating possibilities of the use of chemical and biological weapons. Some day we may possess a weapon capable of destroying all, or nearly all, human life. Would anybody ever use such a weapon? Well, yes. Caligula once said he wished the whole Roman Empire had just one neck so he could kill everybody at once.
It seems almost inevitable that such a weapon will be devised. If it is built, someday somebody will use it. We will either vanish or be so thoroughly decimated that the few survivors will take a long time to return to anything resembling a technological society.
This gives a solid foundation to propose that a species that comes here from another planetary system will not be likely to be intent on our destruction. Those societies that do not learn to abolish their violent tendencies will at some point manage to destroy themselves. Only those who have put aside any use of weapons for the purpose of gain will survive long enough to send US ambassadors.