Hmm, good question – what is up with this ending?
Basically, the story leaves us right in the middle of things. If you're stumped by the rapid-fire sequence of events of the last chapter, don't worry—so are we, and so is everyone. Does the ghost of Peter Quint finally overwhelm Miles and kill him? Does the boy's shock and terror squeeze the life out of him? Does the Governess herself murder him?
James, being the brilliant rascal that he is, obviously intends for his readers to reach the end and be totally confused. The ambiguity of events and of the cause of Miles's death is clearly the key to understanding the actual truth of the story—however, rather than give all of this away to us, James just stops at the point of revealing everything.
This isn't just laziness, folks, it's genius.
Think about it—by refusing to tell his readers what really happens to Miles (and therefore refusing to tell us if the Governess is trustworthy or not), he leaves an indelible imprint on the reader's mind. Instead of sitting back and being told a story, we are forced to engage with it, and to attempt to figure it out ourselves.
While it may be grossly unsatisfying the first time you read it, trust us—the more you mull it over, the more fascinating this ambiguous, infuriating ending becomes.