what is a person who is unusually interested in others' misfortunes
Next to schadenfreude which @Hank has proposed in his answer (prior to this edit of mine) and which, in my view deserves the award of correct answer, the nearest strictly English term is probably busybody, defined by the OED as - An officious, interfering, meddling, or prying person; one who involves himself or herself in other people's affairs without invitation.
It doesn't specify that a busybody is necessarily interested in other's misfortunes though less than well-disposed motivations are often implied by its use. A good example of a busybody is the brilliantly enacted character of Susan Carter in the BBC radio soap The Archers.
The only problem with schadenfreude is that it describes a condition, whilst the OP's request is for the name of such a person.
Perhaps schadenfreude will fit as the word you are looking for:
Schadenfreude
Enjoyment obtained from the troubles of others
A feeling of enjoyment that comes from seeing or hearing about the troubles of other people
(MWD)