schlimm
bad
Describes a negative, unpleasant, or undesirable quality of a situation, event, or thing.
Es ist nicht so schlimm, wir versuchen es einfach nochmal.
It's not that bad; we'll just try again.

bad / serious
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bad
Describes a negative, unpleasant, or undesirable quality of a situation, event, or thing.
Es ist nicht so schlimm, wir versuchen es einfach nochmal.
It's not that bad; we'll just try again.

The expression 'nicht so schlimm' (not so bad) is very common in everyday language. It is used to reassure someone or to present a situation as less negative.
Ich habe den Bus verpasst. – Ach, das ist nicht so schlimm, der nächste kommt in zehn Minuten.
I missed the bus. – Oh, that's not so bad; the next one comes in ten minutes.
A fixed and very common expression is 'im schlimmsten Fall' (in the worst case). It describes the worst possible scenario that could occur.
Im schlimmsten Fall müssen wir die ganze Reise absagen.
In the worst case, we'll have to cancel the whole trip.
In the meaning of 'ernst' (serious) or 'schwer' (severe), 'schlimm' (serious/severe) is often used in a medical context. Typical combinations are 'eine schlimme Krankheit' (a serious illness), 'ein schlimmer Husten' (a bad cough), or 'ein schlimmer Unfall' (a serious accident).
Nach dem Sturz hatte er eine schlimme Wunde am Kopf.
After the fall, he had a serious wound on his head.
This colloquial meaning almost always refers to the behavior of children, or sometimes pets. A fixed expression in this context is 'ein schlimmer Finger' for a person who plays tricks or gets into mischief.
Du hast die ganze Schokolade allein gegessen? Du bist aber ein schlimmer Finger!
You ate all the chocolate by yourself? You little rascal!
When it comes to moral judgments, 'schlimm' (bad/wrong) is often combined with abstract nouns. Look for words like 'Verbrechen' (crime), 'Lüge' (lie), 'Tat' (act/deed), or 'Ungerechtigkeit' (injustice) in the sentence.
Es ist schlimm, was auf der Welt passiert.
What is happening in the world is terrible.
good
'gut' and 'schlimm' can be opposites when you are evaluating a situation, an event, or a person's condition. In that case, 'gut' describes something positive, suitable, or in a good state. 'schlimm' is used for something negative, unpleasant, or serious, for example an accident, an illness, or bad news. 'Schlimm' is often stronger and more emotional than just 'schlecht' (bad). However, if something is 'nicht schlimm' (not bad / not serious), that does not automatically mean it is really good; it only means that it is not so serious.

Der Sturz war nicht schlimm, aber Paul hat sich sehr erschrocken.
The fall was not serious, but Paul was very frightened.
Nach der Untersuchung sagt die Ärztin, dass alles gut aussieht.
After the examination, the doctor says that everything looks good.