"Worse" is the comparative form of "bad" (think of it as "more bad") and is used in direct comparisons involving two people, things, or group of things:

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- Henry is WORSE at Japanese than Peter is.
- This movie is WORSE than the one I saw last week.
- This sandwich is WORSE than anything I've ever eaten.
- Which is a WORSE way of dying: of hunger or thirst?

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"Worst" is the superlative form of "bad." Think of it as "most bad":

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- This is the WORST movie ever!
- Last week's episode had the WORST ending.
- Sadly, Jo's YouTube video received the WORST reviews.