Discuss the ending of the poem.

Samuel Taylor Coleridge’s long narrative poem "Christabel" presents the well-known theme of good vs. evil, but the poem ends with evil triumphing. I don’t believe that’s what Coleridge intended, though.
Christabel, whose very name means "beautiful Christian," represents good, while Geraldine, whose name interestingly means "capable with a spear," represents evil. The conflict between the two is predicted almost immediately when the mastiff watchdog gives "[s]ixteen short howls, not over loud/ Some say, she sees my lady's shroud" (lines 12-13). “My lady" here refers to Christabel, and the shroud portends death.
The actual conflict begins when Christabel finds Geraldine huddled in the woods, supposedly the victim of five soldiers who attempted to rape her. Christabel is praying at the foot of a huge oak tree when she hears a low moan, the source of which she cannot immediately place, and our narrator pleads, "Jesu, Maria, shield her well!" (line 54). This prayer should signal to readers that Christabel needs protecting from something, although neither we nor she yet knows from what. Christabel summons the courage to walk around to the other side of the tree:

There she sees a damsel bright,
Drest in a silken robe of white,
That shadowy in the moonlight shone:
The neck that made that white robe wan,
Her stately neck, and arms were bare;
Her blue-veined feet unsandl'd were,
And wildly glittered here and there
The gems entangled in her hair (lines 58-65).

Because of the other elements in this poem that call upon Christian symbols and acts, it would be safe to say that Coleridge here is evoking 2 Corinthians 11:14, which says, "And no wonder! For Satan disguises himself as an angel of light." Evil would be rejected if it presented itself as ugly and undesirable; instead, it presents as "a damsel bright," not only beautiful but also dressed in robes of purity. The good in Christabel recognizes the evil it faces and cries, "Mary mother, save me now!" (line 69), but she is naïve and her heart is pure and soft; she takes Geraldine home, inviting evil in.
As the two women reach and enter the castle, we have another indication that Geraldine is not what she seems: the guard dog, an old mastiff, growls in her sleep as the two pass her. We are told that this is highly unusual because Christabel is well-known to the dog, and the dog has never before growled in her presence (lines 148-151), but even in her sleep, the dog seems to sense the danger and the evil presence Geraldine brings. In addition, as the two women proceed down the hall, a dying brand on the wall flames up, lighting Geraldine’s face just enough for Christabel to see her eye (lines 158-160), a precursor to a significant later event. As the two reach Christabel’s chambers, heavily carved with various figures, Geraldine falls to the floor at the feet of one of the carvings, an angel bearing light (lines 188-189), and Christabel offers her a potion her mother made that will help her feel better. We also learn that Christabel’s mother died during childbirth, and although Geraldine wishes for Christabel that her mother was there with her, she, with "altered voice" (line 204) and "unsettled eye" (line 208) claims power over and commands the mother’s guardian spirit to leave (lines 206, 211-213). Christabel kneels beside her and invokes heaven, which leads to her tender care of Geraldine, who recognizes the good in Christabel: "All they who live in the upper sky/ Do love you, holy Christabel!" (lines 227-228). This directly contrasts Geraldine’s ‘prayers,’ which lead to rolling eyes, shuddering breaths, and undressing to reveal some unholy mark on her breasts and side (lines 246-253), and despite our narrator’s pleas for the powers of good to "shield her! shield sweet Christabel!" (line 254), Christabel is seduced by Geraldine, who places such a spell on her that she cannot even speak against it (lines 267-268).
In the morning, Christabel wakes from her trance and immediately cries that she has sinned. She hopes her prayers will be heard and that she will be forgiven (lines 387-390), and the two women go off to find Baron Leoline. Leoline learns that Geraldine is the daughter of a former friend and embraces her. During this interaction, we see the evil Geraldine represents. She sees a vision of Christabel’s soul, and her response is to hiss (line 459). In contrast, Christabel is praying, her eyes toward heaven (line 462), and when her father asks her if she is okay, she, still being under Geraldine’s spell, cannot answer anything except "yes" (lines 472-475). Leoline asks Bracy the bard to see Geraldine safely home, but Bracy asks permission to put it off because he has had a vision of a green snake strangling a dove named Christabel (lines 525-556). This is a significant part of the conflict between good and evil presented in this poem. The snake has long been symbolic of evil and in the Christian tradition, of Satan himself, whereas the dove is symbolic of peace and Christ. In Bracy’s vision, evil is triumphing. Leoline brushes off Bracy’s concerns, but Geraldine looks sidelong at Christabel, a look that reveals the snake, the evil:

And the lady's eyes they shrunk in her head
Each shrunk up to a serpent's eye
And with somewhat of malice, and more of dread
At Christabel she looked askance (lines 585-589).

Our narrator again warns us that something evil is coming: "Jesu, Maria, shield her well!" (line 584), but Christabel goes into a dizzying trance, during which she stumbles about hissing (lines 591-593). The spell passes, and Christabel prays, realizes the danger she is in, and entreats her father to send Geraldine away, although because she is still under the spell from the previous night, cannot tell her father why he must (lines 615-622). Leoline’s immediate reaction is confusion because he loves and wants to please his only child, but he also wants to do right by the child of his old friend, and he chooses the latter, becoming angry with Christabel for being so inhospitable, leaving her in Bracy’s care, and walking away with Geraldine (lines 642-657).
Until these last 15 lines, we have the perfect set-up for Christabel to cast off the spell and banish Geraldine or overcome the evil in her through purity, love, and kindness. We see neither scenario develop, and we are left with a puzzling end where evil not only appears to triumph, but even Christabel's father seems to abandon her in favor of Geraldine. There are clues in the poem that suggest that Coleridge, although intending to recognize the power of evil, did not intend for it to triumph. We know that he intended to write three more parts of the poem, but he never finished it, so it would be foolish for readers to take the circumstances of the story as we are left with and analyze them as though they fully represent Coleridge’s intentions. They don’t. Some of the small clues Coleridge leaves for us may signal what his intentions were. First, we have Christabel’s growing awareness of the spell and the danger Geraldine presents. We also have clues that Geraldine herself does not want to inflict evil on Christabel: she seems to be genuinely fighting against harming Christabel but "suddenly, as one defied/ Collects herself in scorn and pride/ And lay down by the Maiden's side!" (lines 260-262). She has a vision of evil she has brought touching Christabel’s soul, and seems to feel regret:

Which when she viewed, a vision fell
Upon the soul of Christabel
The vision of fear, the touch and pain!
She shrunk and shuddered, and saw again (lines 451-456).

After her serpentine gaze causes Christabel to fall into a dizzy, hissing trance, she seems to seek forgiveness from Leoline:

And Geraldine again turned round
And like a thing, that sought relief
Full of wonder and full of grief
She rolled her large bright eyes divine
Wildly on Sir Leoline (lines 594-598).

Careful examination of the poem allows us to see the conflict between good and evil being set up. Looking at these moments and knowing that Coleridge intended to write three more sections for this poem allows us to say that he likely intended for good to triumph.
References:
http://rpo.library.utoronto.ca/poems/christabel
http://www.meaning-of-names.com/