Malignant would be the main answer, but another possible one is Cancerous.
Malignant
Malignant
benign
The word "cancel" is the opposite of sit, in the sense of convene.
The 5 letter word that means the opposite of big and has a short vowel sound is "small."
The prefix ben- means good.
Stand.
often
unsharp
It is thin.
exit
leave