Or they need a new home. In which case, we'd be in their way and would be quickly eradicated.
Stephen Hawking has made a pretty compelling argument that maybe we shouldn't try and contact alien civilizations. It wouldn't be anything like Columbus and native Americans. It would be closer to humans vs. ants. When was the last time you tried to engage in diplomatic negotiations with that fire ant hill in your back yard? You didn't. You eradicated it swiftly without any troubling thought.
This is what aliens would do to us.
In his Playboy interview many years ago, this subject was brought up with Stanley Kubrick. He said, “If an alien ant at my feet scrawls out ‘Let’s talk’ in the sand, I’m not going to step on him.”