How? Well, by refusing to help Young Ben, Jack thinks he is preventing everything bad that happens later/earlier that he blames on Old Ben. Because he doesn't help, Juliet and Kate are forced to take Young Ben to Richard for help. Richard later tells them that, if he helps Ben, Ben will never be the same. That is, the quasi-innocent kid will turn into a serial killing mass murdering liar and thief . . . all because Jack thought he could change the past.
If only Jack had been paying attention when Miles was explaining time travel to Hurley . . .