It's not a good look when you speak ill of the dead, and false platitudes should also be avoided, so maybe he should have just kept quiet. My guess he is seething at the role McCain played in obscuring the origins of the Steele Dossier, and ensuring that it was spread around the media and the FBI. This supposedly kicked off the Trump Russia Collusion narrative, and also led to the special counsel investigation. Of course it was all planned well before, so this process was more about legitimising events. Under the circumstances I think most people would be feeling a little irked.