While not as popular as the debate about whether there are aliens or not, the debate about whether aliens would be hostile or not is rather interesting. Some scientists, such as Stephen Hawking, take the view that aliens might well be like us-that is, rather hostile. Others have claimed that they would be peaceful. These folks often use various stock argument. One is that any race advanced enough to cross the stars would have advanced beyond bad behavior. While the debate is current theoretical, it can obviously be approached rationally.
One approach is the method of analogy. Since we have not encountered any aliens (as far as we know), we have a sample set of one intelligent species, namely our own. Since we are rather hostile, the logical inference is that other intelligent races would also be hostile. Of course, this argument is exceptionally weak since the sample consists of one species on one planet. Other species could be quite different. However, a sample of one is better than a sample of none, hence the best conclusion would seem to be that aliens are probably hostile like us.
Another approach is to consider the conditions that would need to be met for a race to be able to travel from star to star and how this would impact their behavior.
As noted above, one stock argument is that aliens would be peaceful because they would need to be advanced and advanced races would be peaceful. On the face of it, both premises can be challenged. First, it might be the case (as some sci-fi writers have speculated) that interstellar travel can be done with a very low level of technology and we just failed to make that discovery. Second, a race might acquire advanced technology by means other than advancement (such as finding it in a crashed ship). Third there seems to be no correlation between technological level and peacefulness. After all, humans have not shown any tendency to be more peaceful-we just have more advanced ways of hurting and killing each other. While alien races might be different, there is no foundation for the claim that advances in technology must correlate with increases in peacefulness.
Another stock argument is that a race would need to survive past the crisis of self-destruction and also become unified in order to master interstellar travel. Such a race, some argue, would have learned peace. While this has some appeal, the argument is easy enough to counter. First, there are various ways a race could get past the crisis of self-destruction without being peaceful. For example, if the race was unified by war prior to this crisis it could still be quite hostile. As another example, a race that is far more collective than humans could be unified, but unified against the rest of the universe in a very hostile way.
Another stock argument is that interstellar conquest and war would not be feasible because of the distance and cost. This, of course, does not show that aliens are not hostile-it just shows that they would have little or no way to act on that hostility. However, it is easy enough to imagine ways around these problems. Perhaps war and conquest would be feasible. After all, getting an entire planet would probably be worth the cost of getting to it and conquering a world might require sending only one ship and the right equipment (like an automated factory that could build a robotic fleet and army on Mars and attack earth with locally built forces). Another possibility is a race that is desperate and needs another world to survive, regardless of the cost (which is a classic sci-fi plot device). Or perhaps there are races that would send a weapon to exterminate us, perhaps out of xenophobia.