Christopher Lee gives us an absolutely perfect Motive Rant as King Haggard too. A Haggard says that he thought he would die when he saw his first unicorn.
As noted elsewhere, Lee read the book and loved it so much, he underlined entire sections which he insisted be kept in the film, including the Motive Rant. Without him, it risked being left by the wayside.
Both the film and the book
Molly Grue's reaction to Schmendrick celebrating he was able to turn a Unicorn into a mortal woman.
Schmendrick:(capering happily) The magic chose the shape, not I. I am a bearer, I am a dwelling, I am a messenger!
Molly: You are an idiot!
When the Unicorn sees Prince Lir's Heroic Sacrifice and decides to charge the Red Bull instead of running from it. She is the first unicorn who had something worth fighting back for. To say nothing of when the Red Bull turns and goes into the sea, defeated — and then a HORDE of unicorns comes charging up out of the sea, running all around Haggard's castle... which then falls apart and collapses into the sea, with King Haggard falling to his doom shrieking in bitter triumph, "THE LAST! I KNEW YOU WERE THE LAST!"
Lir: Do something, you have the power! I will kill you if you don't do something!
Schmendrick: I cannot! Not all the magic in the world can help her now!
Molly Grue: Then what is magic for? What is the use of wizardry, if it cannot even save a unicorn?
Schmendrick: That is what heroes are for.
Lir: Of course, that is exactly what heroes are for!
One for the animators; after Lir falls to the Red Bull, the Unicorn's whole demeanor goes from one of fright, to looking at the Bull as if to say, "You son of a bitch!" The animators capture it perfectly.