When worse gets worse, what have you got? The worst! In this phrase, we’re going from a comparative adjective (X is worse than Y) to a superlative adjective, which conveys the upper, or in this case, lower limit of a quality (the quality here being badness.) So the correct expression is: “if worse comes to worst.” As in, if the worst thing happens.
(And while we’re here, it’s worst-case scenario, not a worse-case one.)