If it wasn't on purpose, you didn't have to go away. If it hadn't been on purpose, you wouldn't have had to go away. In the first sentence, I interpret this as the speaker not knowing whether or not the other person's actions were deliberate and informing the other person the connection between doing it on purpose and having to go away. It's a hypothetical discussion and there is no indication of whether or not the other person did actually go away. In the second sentence, the grammar tells the listener that the other person's actions were deliberate and they did go away. Again the speaker is explaining a connection between the action and having to go away, but the action has happened and the other person did go away. I hope that helps answer your question.

Elijah explained the difference, I would probably include 'then' to make it sound more like a cause and effect/if then statement. You can include 'done' to 'done on purpose-- this may make it a but more clear; alternative to 'go away,' you could just say 'leave.' Those are just stylistic preferences, but to me, they make the message clearer. 'If it wasn't done on purpose, then you didn't have to leave.' 'If it hadn't been done on purpose, then you wouldn't have had to leave. In Russian, I think the second would be more of a бы hypothetical--i.e. if the person hadn't done whatever they did on purpose, then they wouldn't have had to leave. Whereas the first is agnostic to whether the 'thing' was done on purpose or not. I'm not sure if the below translates in Russian in the same way: 1) Если это было сделано не специально, то тебе не нужно было уходить. 2) Если бы это не было сделано специально, тебе бы не пришлось уходить.