I have met devils. They are not so uncommon in our world. Cambions -- creatures born from humans and devils; are able to easily exist in both worlds, yet they wield much of the powers that full-devils do. And Avernus, being the first of the hells, often sends its fiends into our world. Though I believe perhaps Zariel holds grudges against humans for a past betrayal... the specific accounts conflict much in these details.

[ Now there's a curious question. ]

Why, that is simple. Those who enter the Celestial plane are those whose lives were judged as just. The god I sword my Oath to, Tyr, believes in justice, honesty, and mercy. Those who are aligned with the good are honorable. And those who are aligned with evil, believe in retribution, self-preservation, and dishonor. An unlawful life sees one banished to the Hells, though if one is lucky, one may slip in between perhaps, and end in one of the other realms. It is largely dependent on which god, if any, speaks for one's soul upon one's death.