Student or Learner
If someone is hanged or if they hang, they are killed, usually as a punishment [...]
(Collins Cobuild Advanced Learner’s English Dictionary)
From this I've drawn a conclusion that the following two examples should mean exactly the same:
1) They'll hang.
2) They'll be hanged.
But is there any reasonable explanation why and how 1) has found its way into the language?
It feels to me like it's just a difference in whether you're thinking of someone actually hanging the person up (be hanged), or just the person is up hanging on a rope (hang).
They'll be hanged (by somebody for some reason)
They'll hang (for some reason)
And you're right, the two mean exactly the same. You can hear in movies "You'll hang for this/I'll see you hanged for this."
(not a teacher, just a language lover)