Arnold is harming the average score of this movie.
But not in the most common way. His performance was great. He was very good in this film. He conveyed the character perfectly.
But what is happening is that many of his fans were expecting to see him cleaving zombies left and right (many obviously didn't read about what this movie was). And then they got this sort of slow paced drama about family bonds, disease and how society deals with the hopeless contagious sick.
Of course this will result in people coming here and bashing the movie and giving it a poor score. This was marketed to the wrong crowd. And it shows, both in ratings and reviews.