worst, being the superlative form of bad, is always preceded by the definite article the.

the expression the worst came to the worst is fixed and can not be altered.

worst occurs in semi-fixed expressions, such as if (the) worst comes/came to (the) worst.

the only instance in which worst is not preceded by the definite article is when it occurs in the expression at worst.

the expression worst of all always occurs at the beginning of a sentence.