That was not stated at all, we could have quite easily managed to hit Greatness 10 with the XP we generated and never hit Heartlessness 10.
We hit Heartlessness 10 because Odyssial was prone to taking very, very efficient steps to getting what he wanted, which often paved over the interests of others. He could spend more time on megaprojects because he spent time crushing anyone who would get in his way.
NPC don't get EXP. NPC would gain or loose Heartlessness by their deeds and what influences them.