Uh, it has been a long time since I saw the movie, but here is what I remember.
The aliens they encountered at first were only like a recon team, they weren't going to invade, or even attack humans. However since the main character chopped off the one aliens fingers, he decided to go back and get revenge on the one family.
However, there was a flaw in that also. The whole movie was based off of predestination. So supposedly the wife had to be killed, to tell the older son to swing away, causing him to remember that when the aliens attacked and thus saving the family. However, if the wife would have never been killed, the main character would never have had to go to Vet's house and would have never cut off the aliens finger causing the attack and everything would have been avoided.