Information is always, or almost always, better in the brutal daylight of the public sphere. The Man made the same argument about some of Ted Kaczynski's writings, that it would inspire copy cats, etc. It hasn't, and his writings are fascinating and borderline prophetic.
Lanza was a failure in life and his writings should be available to others because they, better than almost anything else, will make plain his failure and self-delusion.
Is that a really fair comparison though? Kaczinski was a self sufficient, well educated genius; Lanza lived with his mother and was a college dropout. Potential Kaczinski's are far rarer than potential Lanza's.