The roque mallet lends the story an unrelenting and murderous rhythm, creates suspense, and works as a rather unique symbol. We hear about the roque mallet from the first Tony incident and then we never stop hearing about it. Danny has frequent visions of some creature (which, of course, is Jack) using the mallet as a horrifically destructive weapon. In a way, Danny's predictions come true. Jack mallets Wendy every which way, and he also mallets Halloran in the face, temporarily halting the rescue mission. As predicted by Danny, Jack also uses it to destroy the caretaker's quarters at the Overlook, along with Wendy's records.
But, the part of the prophecy where Jack mallets Danny is not fulfilled, as we discuss in "What's Up With the Ending?". Instead, Jack (who the narrator is now calling "it") turns the mallet on himself, as we see in this excruciating moment:
[…]instead of aiming at Danny, it reversed the handle, aiming the hard side of the roque mallet at its own face. […] Then the mallet began to rise and descend, destroying the last of Jack Torrance's image. […] What remained of the face became a strange and shifting composite. […] Danny saw the woman in 217; the dogman; the hungry boy thing that had been in the concrete ring. (55.76-78)
In a combination of the extremes of self-loathing and extreme supernatural influence, Jack Torrance is completely taken over by the Overlook at this moment. When we look at Jack's earlier musings on roque and the roque mallet, we can see the ironic symbolism at work. While in the equipment shed where he first encounters the roque set, he thinks,
"[…] it was a schizo sort of game at that. The mallet expressed that perfectly. A soft end and a hard end. A game of finesse and aim, and a game of raw bludgeoning power." (33.6).
Schizo here is the pop-psychology abbreviation for schizophrenic. Here, King is playing on the idea of the split or divided personality who loses time and who can't accurately perceive reality. If you discount the supernatural or some reasonable explanation for the scene at the Overlook, you can chock it all up to schizophrenia. Furthermore, Jack Torrance is a tad deficient in finesse and aim these days. He's leaning hard toward the raw bludgeoning power side of life. Even his "soft side" is capable of doing damage, or of coming under the control of his violent side. The mallet embodies the confusion within Jack, with regards to the different sides of his personality.