The purity of purpose got John into hell in the Chpater 2. They admitted they can't find anything else as pure as the original, so they find the other way out, They don't want to take another dog or love interests away from John just for him can have some pure vengeance again. It's a choice, not a flaw.

Of course there's no clear purpose in the 2nd one, because the 2nd one is just the consequence of the purpose fulfillment from the 1st. It's actually mocking at you because what you loved about the 1st one, the purpose or the vengeance or whatever, is blowing up in front of John's face in the 2nd one and John is…