On its own, it was fairly good; not very much to say since I couldn't help but think about the context. The ending was a little confusing, but if you saw the movie you'd probably be surprised if it wasn't confusing.
Now for the context. No one's first film is great or even good, but this was a very good first effort from the writer-director and I think it showed a great natural artistic capability that a longer career would have honed. I was amazed at how good this was for a first film and it's tragic to think there will never be another movie from this clearly gifted artist.
That being said, if I didn't know the backstory behind the film, I probably wouldn't have sought it out, let alone even heard of it, and it's better to judge a film on its own rather than the context around its making. I acknowledge that I had a bias going into this movie as I went in knowing the tragic story of the writer-director, but the bias should not be judged too harshly as I've gone into many movies wanting them to be good and that's never stopped me from judging them to be bad if they were indeed bad. While The Evil Within is not great, it's certainly memorable. 6/10 might not sound like a good rating, but I prefer a 4-star scale, with 2 stars being average, not bad but not exactly memorable, and 3 stars being good, and this movie's in between, so my rating is 2.5/4 stars. A not exactly good but certainly memorable movie, and an incredible first and only film from an untapped talent.