In South Carolina deadly force may be used to prevent harm, if one deem it necessary to prevent harm. There must be a reasonable fear of immediate danger for this to apply. This applies to defending someone else.
To prevent trespassing one only needs to convey that the way they dealt with the trespasser was appropriate.
Castle Laws are laws that address the use of force when defending one's self inside their home, or on their property. Some states expand this to vehicles, and the person's place of work. Castle Laws generally include: the places where this law applies, the requirements fro use of deadly force, if there is a duty to retreat, the amount of force that maybe used in defending one's self or others. These vary widely from state to state.