Improper means of acquiring trade secrets include theft, bribery, lying, breaching or inducing someone to breach a duty, and espionage, but it does not include reverse engineering, independent derivation, or other lawful means of acquisition. 18 U.S.C. § 1839(6).
Someone else breaching a contract does not make your use of the information misappropriation if you did not induce him to break the contract.
Improper means do not have to be illegal, but anything illegal is improper means.