For example, “flammable” and “inflammable” both describe an object that can easily catch on fire. I can also think of “ceased” and “deceased”, both of which can mean someone or something has been brought to an end.
edit: Some people are including words that can also mean its opposite (like sanction or table), those are cool too! The more weird words, the better!
This is kinda the opposite to what you are asking: awesome and awful used to mean the same thing IIRC, both being something filling you with awe. Later the meanings split between positive and negative.
Same with terrible and terrific.
As a non-native English speaker, I always have to think a second extra about “terrific”, about it being negative or positive. Probably because it sound similar to “terrible”.
Or as an analog: horror - terror, horrific - terrific.
You’re right to think that, because that’s how both words began!
https://www.etymonline.com/word/terrific
I often thought awful was an odd word. Surely awful = full of awe, but it is frequently used to describe things that have little to no awe.
Also I have now said the word awe too much and it looks strange.
My guess is it came about because people were using it to mean “something so bad it fills you with awe”. Like “the thing humans are doing to the environment are awful”. But then it lost its connection to awe.
That’s my guess.