I can't avoid this anymore: Graham Crowden gives one of the more unforgettable performances in this story in anything. Ever. Simultaneously channeling Ming the Merciless, it seems, and every other over-the-top cartoon villain ever created in modern science fiction, Crowden's Soldeed really must be seen and heard to be believed. His famous death scene, which Crowden apparently didn't know was the actual take, and thus laughed maniacally at the end, actually isn't out of character at all, because I could totally see Soldeed doing exactly that.
The fact that Soldeed's body disappears in the next scene, though, is a bigger bone of contention. It is just one of many lazy directorial flourishes from first (and last) timer to Doctor Who, Kenny McBain, in a story that has been, if it's possible, over the top in its laziness. There is no restraint seen in any aspect of this story - everything is blown so far out of proportion and scale that there is absolutely nothing believable in any of the characters actions or motives. The worst thing possible is happening here - Doctor Who is being played for laughs, and there's no one around to stop it. Certainly not the producer, the director, or the leading actor. This story, above any other, shows the distinct need to stop the madness that Doctor Who had become and pull in the reins to make respectable science fiction drama again.
In short, The Horns of Nimon is possibly the most ludicrous story ever produced, and another one best viewed in retrospect. At the time, it was probably looked upon as laughably bad. Now? It's laughably good. Not just for its faults, but because of them, you may never have a more enjoyable time watching a Doctor Who story.