Great Movie but there is a doubt about an incident

Rating: 10/10 (24 December 2015)

This is a great movie, and I watched it many times -- with every time I watch it , my love and understanding of the movie grows.
One of the best movies involving jail terms.Not only have a strong story line, but also backed up by great direction and very strong acting.
The voice of Morgan Freeman is a catalyst.
There is one part for which I did not get an explanation from the movie -- why did the warden kill Tommy Williams? Of course, Tommy Williams could prove that Andy was innocent, but what benefit would the warden get by stopping him? I did not find any reason for that act.