John Kreese explained (the evil Sensei from The Karate Kid)

I beleive John Kreese, was sent to Vietnam at a young age, as a young man he was idealistic and had compassion for people. When he first saw combat, he had the oppurtunity to finish a wounded enemy soldier, instead he showed mercy and let the man live, immediately the man pulled a hand grenade and killed John's best friend. He realized that his weakness had gotten his friend killed, this angered him and plagued him with guilt, he then came to a conclusion, I must kill the enemy, or he will kill me. This is where it came to Kreese that "mercy is for the weak." When John came home from the war he learned Karate, during his study he was taught that Karate is used for self defense. John never changed the philosophy of Karate, he just knew from practical experience that shooting a sniper before he shoots you was the best defense. This is the foundation of the Cobra Kai, the best defense is a strong and punishing offense. John loved his Cobra Kais, and that is why he was so tough on them, he wished his father was tougher on him when he was younger, so that he would not have had to grow up so quickly one fateful day in Vietnam. When you watch Karate Kid, do not judge Kreese as some brutal man who gets enjoyment out of watching a 15 year old kid get beaten, understand although his intentions and actions may seem misguided, his heart is just that of a patriot. He truly beleives the beatings Daniel takes will make him a stronger man, the ending of this movie is proof that Kreese, although portrayed as the bad guy, truly is the most motivating factor behind the whole movie. Ultimately and indirectly Kreese provides Daniel with friendship and self confidence that would not have happened otherwise.