As suggested by the flavor text, each word in the left column has an antonym which is also an antonym of a word in the right column. For example, HARD is the opposite of both EASY and SOFT. As added confirmation, if you connect these pairs of words, the line will go through letters which spell out the shared antonym, minus one letter.
The missing letters, in order of their left-column words, are AEWGYVAI. Since the words on the left are given in alphabetical order, it makes sense to reorder these letters by putting their right ends in alphabetical order. This gives the answer GIVEAWAY.