We are getting into unknown territory. But the traditional belief is that Purgatory is the place where venial sins and the remaining effects of confessed mortal sins are purged away. If you die with mortal sins on your head, you are in trouble. And there are few worse sins than being responsible for the murder of thousands of innocents.
Even this could be confessed and repented. I have no idea what the state of Kennedy's soul is, but it's also a traditional belief that if you confess sins with no real contrition and no intention of amending your ways, the confession is valueless. Kennedy obviously intends to keep voting for abortion every chance he gets, so it probably would do little or no good for him to confess his past sins in this area until he resolves to change his ways.