It means "John Lennon is
dead". Alternatively, 'john' can be substituted for any other name to create the same effect
Originating from 4chan, "john is
kill" was the assumed 'title' of a thread about the assassination of John Lennon seemingly typed out by someone with very little English skills, to comedic effect. The OP was asking
people where they were and what they were doing when they heard about John's assassination. Now the thread is sometimes recreated using the same picture of John and using the same format, although sometimes Heath Ledger is used instead (or
heat ledger, as it is typed out in homage to the original
OPs poor English)