Which is correct: worse comes to worst or worst comes to worst? The former seems more logical but the latter is what appears in Oxford Advanced Learner's Dictionary.
What is the word for a cycle where the cause is made worse by the effect? A very simple example is a decline in fish leads to a decline in coral reef health which then leads to a further decline in fish and so on.
The idiom is "for better or worse" - is it this you mean? If so, it just expresses this: If a situation exists or happens for better or for worse, it exists or happens whether its results are good or bad. See here. This comes from an idiom "for the better/worse", which means "to produce improvement/decline". If this is not what you are looking for, there is a phrase "for the better part ...