Student or Learner
"It's dangerous, back up!".
Is a full stop needed?
It's dangerous -- back up! - I would use a dash because "back up" is an added expression that connects to "it"s dangerous" logically. It does not, however, connect in a grammatical way to "it's dangerous", and this is where I find a dash to be a good choice.
I think using another exclamation would be good as well.
It's dangerous! Back up! - This can easily be read as two exclamatory sentences.
To the exclusion of errors, punctuation sometimes has to do with personal preference. In other words, there's more than one correct way to punctuate in some instances.