He came from a different era, where "masculinity" was determined by your aggression and a sort of survival-of-the-fittest attitude, where fighting (be it in your personal life or your professional life) was the way to prove your worth. This is exemplified by his infamous "real power is something you TAKE!" motto. So it's no surprise that he would think that the best way to make his boys strong is to have them fight each other, to see who wants "it" more. In his mind, this should bring out the best in both of them. And it's why he ultimately rejected Gary, as he didn't have those aggressive tendencies, and Jock saw that as "weak".