Stephen Hawking has mentioned that aliens, if we find them, may be dangerous. The news has latched onto this as if what he said wasn't obvious to anyone with any passing knowledge of social contact of any sort.
Hawking rightly warns us of the lessons learnt in history time and time again when a more powerful civilation encounters a less powerful one. The results, to date, have rarely been pleasant for the lesser* civilisation.
Some people may say that advanced civilisations must surely be beyond such violent behaviour as would be demonstrated by an annihilation or enslavement of the human race. I rather think perhaps the Aztecs might have thought the same before their civilisation was reduced to a shadow of its former self by the conquerors they initially mistook for gods.
There is no guarantee of what any other alien life we find would be like. We don't know what they would look like, what environments they would live in, how advanced they would be, how they would communicate, what their motivation would be if they contacted us at all or or indeed whether we would ever be able to meet in the first place.
I would suggest that anyone with an overly optimistic view of what other alien civilisations must be like, if they exist at all, should read or watch a little more science fiction.
Start off with Alien.
* Lesser only in terms of the ability to defend itself.