Worst is the superlative of bad.
Worst is the superlative of badly.
Worst is used to form the superlative of compound adjectives beginning with 'bad' and 'badly'. For example, the superlative of 'badly-affected' is 'worst-affected'.
The worst-affected areas were in Jefferson Parish.
受影响最严重的地区是杰斐逊教区。
The worst is the most unpleasant or unfavourable thing that could happen or does happen.
Though mine safety has much improved, miners' families still fear the worst.
虽然矿井的安全状况有了很大改善,矿工的家属们仍然害怕最糟的情况会发生。
worst of all
You say worst of all to indicate that what you are about to mention is the most unpleasant or has the most disadvantages out of all the things you are mentioning.
The people most closely affected are the passengers who were injured and, worst of all, those who lost relatives.
受影响最大的是那些受伤的乘客,那些失去亲人的乘客尤为甚。
at (the) worst
You use at worst or at the worst to indicate that you are mentioning the worst thing that might happen in a situation.
At best Nella would be an invalid; at worst she would die.
内纳往好里说也会落下残疾,往坏里说可能会性命不保。
at one's worst
When someone is at their worst, they are as unpleasant, bad, or unsuccessful as it is possible for them to be.
This was their mother at her worst. Her voice was strident, she was ready to be angry at anyone.
这是他们的母亲情绪最坏的时候,她说话咄咄逼人,随时会对任何人发火。
if the worst comes to the worst
You use if worst comes to worst or if the worst comes to the worst to say what you might do if a situation develops in the most unfavourable way possible.
If worst comes to worst, Europe could withstand a trade war.
在最坏的情况下,欧洲还是可以顶住一场贸易战的。