Actually, that really isn't too hard to believe. The evidence of the kidnapped girl leads me to believe he has some sort of psychosis. I dont think the invasion/abduction was a coincidence though, I think it was more opportunistic on Howard's part. They were in the same area by happenstance when the emergency broadcast went out, but Howard seized his opportunity to grab a valuable necessity: a female. If the end of the world were to occur, you would need to have both males and females in order to repopulate, and Howard being in the survivalist mindset and always prepared wouldn't have over looked this.

Howard's view of Michelle is much like that of an abusive partner, seeing their spouse as an object and not a person; something to hold dominion over. The domestic abuse parallels are pretty blatant, but I don't believe it acts solely as a trope, I do believe it furthers the storyline of Howard being in the survivalist mindset, though it draws inspiration for their relationship from abuse. Another huge detail that points toward Howard "collecting supplies" is the state in which he kept his previous captive, he didnt murder her, and allowed her to grow discontent until harming herself in an attempt to call for help. Also, what he says to Michelle after shooting Emmett: "This is the way it was always supposed to be, you're safe. Now it's just you and me." His plan was never to kill her, or to kill the previous captive, it was necessary to have a male and female.

Also, the broadcast at the end of the movie is claiming they have "taken back the southern seaboard", as a bolt of lighting lights up the sky revealing 2 ships, 1 quite a bit off in the distance over the city she's driving toward. Cloverfield had a similar ending, showing a brief home movie at the end, but revealing a space ship crash landing in the ocean. I think they might be foreshadowing future events, and perhaps the recording isn't coming from survivors, but the aliens in an attempt to gather survivors.