Do I need Steam for Kerbal, or is it an option that you can choose not to purchase/load/use?
Can I play Kerbal without Steam? Yes, to answer the question in your title.
Do I need Steam for Kerbal? No, to answer the question in your post.
Steam is not required for KSP. When you purchase it from the Kerbal Space Program store, you either buy a Steam Key, or buy it so that you download / patch it from their servers. This is a choice you only get to make once.
Just click on "Add to Cart" rather than "Buy a Steam Key" and you can continue your blissful Valve-free rocket experiences.