The word benign can be defined as referring to anyone showing "gentleness or kindness," such as a "benign king" (Random House Dictionary). It can also refer to anything that is favorable or demonstrates both kindness and gentleness, such as a "benign smile" or a "benign omen" (Random House Dictionary). Therefore, any antonyms for benign would be the exact opposite of anything kind, pleasing, or beneficial.
In Lois Lowry's The Giver, the word benign is used a few different times, such as in Chapter 8 when the community is described as feeling somewhat relieved by the Chief Elder's "benign statement" that she has not made a mistake in omitting to give Jonas an assigned job within the community because she has selected him instead to do a more significant task (62).
Using a glossary from The Giver, we can also find a couple of different antonyms for benign. One example of an antonym would be fretful, used in Chapter 12 to describe Gabe as being "so fretful at night" (91). Since fretful means being full of "worry" or discontentment, we can easily see how being fretful is the exact opposite of being benign (Random House Dictionary).