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Actually, Stephen King gave his go ahead with the ending, which is completely believable. One of the major underlying themes in the movie is how far people can be pushed before resorting to extremes. You are right, he puts his kid above everything else.
Now, I am not a parent, but I can imagine that if you were faced with letting some other dimension creature possibly tear your kid to pieces or saving him from a horrible death by ending it yourself, what would you do? In real life, every day, there are cases of parents who did just that for their children and we call it murder. Now, yes, it's murder, but it's understandable under extreme circumstances such as created in the movie.
All he was doing is trying to spare his son the lesser of what he thought were the only two evils he had. Though, I will note for those of you who have not read the novella, that there is no actual ending, so the movie provided one moreso for closure than anything else.