Worse is the comparative of bad; worst is the superlative. The phrase "if worst comes to worst" is illogical, but that's the way it should be said, provided one wishes to use such a hackneyed expression at all. Informal, slangy, or trite expressions to avoid include "worse and worse," "in the worst way," "get the worst of it," and "at the worst." In the sense of "more," worse is not fully accepted; nor is worst in the sense of "most." Avoid such statements as "I dislike insects worse than I do snakes." "Worst kind" and "in the worst way" are slang for "much" and "very much." "She would like to aid you in the worst way" is ambiguous. If you revise the statement to "She would like the worst way to aid you," you may have added suspicion of immorality to plain confusion.
Dictionary of problem words and expressions. Harry Shaw. 1975.