I agree that Johnny did it to save his friend because he was about to die and he saved a good persons' life. Also, Bob did something bad too, so he is not totally innocent so I agree with you and think that he is justified.

Johnny was justified because if Johnny used his blade when Bob did not do anything or was innocent, that might not be justified but Bob and his gang drowned Johnny on purpose and Johnny used a blade to defend himself and he wouldn't have known that he would kill him. If Johnny didn't do anything and didn't defend himself from them, he would have died.