The situation in question is similar to a scene from the TV show Burn Notice.
Micheal is being held hostage, at gun point, by an old enemy, Larry. Fiona rightfully believes that Larry is about to kill Micheal. Fiona attempts to kill Larry with a rifle but discovers the buildings windows are bulletproof. She then decides to place a small C4 explosive device on the window near Larry and detonated it, killing him and saving Micheal. Assuming Larry's death is, and can be proven to be, self-defence/defence-of-others, what would Fiona be guilty of? I believe C4 is illegal to buy, own and use by civilians, so even with it being a lawful killing, what would Fiona be guilty of?