Maybe overhearing that confession/rejection unlocked some 'power' in her to be able to haunt him or so (or maybe she possessed a neighbor to kill him in his sleep?)
... he appears to have been stabbed. How did it happen? Could she have possessed someone? Did she possess him? ...eh, well, cheers to the happy couple.
This reminds me of an old oneshot with the exact same premise but significantly better, this one seems much smaller in size and feel like it has been made to be digested by younger audience.