The antonyms for "setting upon" that come to mind are "leaving alone," "ignoring," and "neglecting." Instead of attacking or ambushing, setting upon, one could choose to walk away, showing a sense of indifference toward the person or thing. By ignoring or neglecting, setting upon the target is not an option, and, more importantly, the aggressor can avoid the consequences of their actions. However, it's important to note that choosing to leave alone or neglect something is not always the right answer. In some cases, taking action, even if it involves conflict, may be necessary to protect oneself or others.