Yes, rock salt can kill tree roots if it is applied directly to them or if it is used excessively in the soil around the tree. Rock salt contains high levels of sodium, which can dehydrate and damage the roots, eventually leading to their death. However, it's important to note that using rock salt to kill tree roots is not a recommended or commonly used method. The application of rock salt can have unintended consequences, such as harming nearby plants, affecting the soil's fertility, and potentially destabilizing the tree's overall health and stability.