Aliens that want to take over the world? Sounds like bad ones to me. There's a lot of good in the universe and even in this galaxy, there has to be. We're not alone in the universe for a generally benevolent race. This is what is hinted in Ancient Aliens, the History Channel series. What a great show it is!
Well, it stands to reason that if there's one race of aliens there's many. And if there's many it stands to reason they would have different motivations and goals.
Just look at humans. We are all the same species and different groups and individuals have wildly different agendas. Some want universal freedom, some want to enslave everyone else and legions of people who's agenda/goals fall in between the two extremes.