Translation:We imagine immediately what is the worst!
You are right, if you take ”mai rău” separately it should be translated ”worse”. But in this phrase, you can see that there is no term of comparison: ”mai rău decât...” (worse than). That means that it is actually a superlative relative of superiority: ”cel mai rău” (the worst). The sentence is elliptic of ”cel”. It could also sound like: ”ce este cel mai rău”.
I think I see what you mean, but how is one supposed to deduce that the phrase is elliptic of "cel", & not, say, "decât aceea"?