Myśleć is non-reflexive, so the unstressed reflexive pronoun się is misplaced here. You can use the stressed dative-case reflexive pronoun sobie after the subject (ludzie), but that's more of a causal speech filler which doesn't make a lot of difference semantically.
Ludzi is accusative/genitive, but since it's the subject here, it needs to be in the nominative case, which is ludzie.
Myśleć o takes locative, so śmierć should become śmierci.
All in all: Niech ludzie myślą o śmierci.