Does Worst Case Have a Hyphen?

'worst-case scenario' ('case' isn't a participle). However, we don't hyphenate those really common or industry-specific phrases like 'best practice standards' or 'lower income earners'.

Which is correct worse case or worst case?

Is it a worse case scenario or a worst-case scenario? The correct usage would be worst-case scenario. A worst-case scenario is characterized by the worst of the possible foreseeable circumstances. It is the most extreme scenario.

How do you know when to hyphenate?

Generally, you need the hyphen only if the two words are functioning together as an adjective before the noun they're describing. If the noun comes first, leave the hyphen out. This wall is load bearing. It's impossible to eat this cake because it is rock hard.