It's definite (that) is omitted here, "something (that) you know is wrong." . Is there a rule for this kind of omission like "If there's a word like "something", the subjective relative pronoun can be omitted" or was this done randomly by the writer's style?
gu56)Your peers don't always encourage you to behave in positive ways....Practice what you'll say if someone wants you to act irresponsibly...And finally, be proud of your strength when you choose not to do something you know is wrong.
Here's my opinion, and it's no more than that -- a quick opinion.
"That" is usually omitted in speech, and quite often in writing too. For myself, I have a tendency to use it in writing, but when reviewing and editing I often delete it as unnecessary. Very rarely I insert it if I feel it clarifies meaning or improves euphony.