Answer to Question #321533 in Law for goddess

Question

X1, X2 and X3 are crew on a whaling boat. A and B are fanatical members of an organisation that campaigns for putting an end to whaling. A and B continuously ram their boat into the whaling boat in an effort to damage it. X1 blasts A and B from their boat with his boat’s water cannon. A and B fall into the water. A drowns. In the meantime, B gets hold of a lifebuoy, and hangs on to it. X2 and X3 swim up to the lifebuoy, and try to hold on to it. The three men start sinking. B and X2 both hit X3, who cannot swim well and is very overweight, until he releases his hold on the lifebuoy and drowns. X1, X2 and B are saved. Answer the following questions: 

(i) X1 is charged with the murder of A. Briefly state which ground of justification X1 can rely on and give reasons for your answer. (Do not give the definition of the defence and do not discuss all the requirements of it. Confine your answer to the relevant requirement/s of the defence.) (3)