BackFire
Blood. It's nature's lube
To be fair, 'Don't Tell Mom The Babysitter's Dead' did deal with someone dying, so maybe there is some truth to the idea of babysitting results in death.
See, the thing is that ending wouldn't have been unique, all movies, particularly thrillers, seem to force themselves to have some surprise twist ending, with little value beyond simple shock. The ending you suggest strikes me as falling into that catagory. Beyond being kinda surprising, what would it add? In fact, the current ending is actually quite unique in that it didn't try and have some mind blowing twist to rely on, it just had a coherent ending that went with the tone and flow of the movie.
Your ending would also result in many questions such as 'why did the neighbor kill that girl to begin with?' or 'why did the girl think her neighbor killed her?" Where did she get the information that this man was actually involved in the murder, since in the film the only reason she finds him is because his partner in crime ratted him out."
Though, one thing is true, it probably would have been a bit better had she actually castrated him.