Doesn't it appear that Jacob was a bit too casual about the whole being killed thing. Jacob almost welcomed it and approached Ben. He advised Ben of his ability to choose, almost as if he wanted to make sure Ben killed him fair and square and not under duress. Jacob almost taunted him with the "what about you?" comment. Perhaps when Jacob or MIB die they become free. MIB and Jacob appeared to be friends maybe trying to kill each other (or at least MIB trying to kill Jacob). Perhaps MIB was trying to kill Jacob at Jacob's request. MIB told him, "Do you know how bad I want to kill you." He never said anything about hating him. Jacob even offered him a meal (albeit apparently sarcastically).