A manga character says this to someone who told him to
言われなくてはならない and if it is, how can it be part of a question (with "nan de")? I don't really understand this sentence at all.
Here's a breakdown of that sentence:
The sentence itself does not define who is being told やめろ、so if the person being told やめろ says this as you indicate, it would mean:
If the other person is doing something that the speaker doesn't like, but he/she doesn't stop until told to by the speaker, the same thing would mean:
First, understand that やめろ is a really raw/strong/rude way to say "Stop (it) !" so I won't translate it to keep the strong meaning. (Other answers are missing this important fact. The speaker does not complain about being told to stop, but the way he is being told to stop.)
With your context, this sentence would mean: Why do you have to go this far by telling me やめろ. (ie: You could have been more gentle, a softer way to say this blabla....)
It's kind of Why do I have to be told things this far/rude as やめろ.