Not for those who like a clear cut plot (spoilers)

Interviews with the director:

https://www.thrillist.com/entertainment/nation/it-comes-at-night-movie-ending-explained

http://www.slate.com/blogs/browbeat/2017/06/09/it_comes_at_night_s_director_on_the_movie_s_ending.html

Basically the movie is designed to be ambiguous. Nightmare, monsters or just the decay of humanity after a virus outbreak - up for the audience to decide. The whole point was that it wasn't supposed to be wrapped up in a little bow - the director acknowledges audiences are finding this frustrating - but he wants it to be a film that can be revisited, and re-interpreted. That in itself is very cool. Personally I liked the concept but it didn't really engage me, it's not a particulary new idea and I found it pretty depressing to be honest, especially when you realise that the big bad is 'within', and the title and trailer are misleading. Kelvin Harrison Jr was excellent though as the tragic Travis.

I go with the theory Travis was sick from the start of the film as nearly the entire movie is seen from his viewpoint, whether in his nightmares or reality, and even his waking moments are filled with cold fear and sense of impending doom. I think he killed his dog in a disease-induced fugue, although his parent's unnecessary killing of a young innocent family is perhaps the most brutal act I've seen in a movie for a while, and I felt their own submission to the virus at the end justified.