I suppose that her lineage to a demon was enough to make her go to the Hellish Yard

But is unfair...

Well, the Third Period never was a really fair realm

Correct me if I'm wrong but wasn't she born a contractor because of her lineage?

But what about the children of venomania who also have demonic lineage ?? Kayo only demonstrated HER behavior after the entire incident, and even then it was to save Onigashima

I guess it's because Venomania descendants are children of a demon contractor, not a whole demon, like Rahab. Also, as I understand, the reason why someone goes to the Hellish Yard and can't cross to the Heavenly is that they are HERs (in the case of contractors, they mingle their soul with the demons who are HERs). Though, I could be mistaken.

That would mean that kayo was born an HER for being the daughter of a demon, but that demon contractor's descendants get a chance of being safe of HERs.

Maybe you should ask Price, surely they have a better understanding or idea.

I think Price already answered this but I wanted to add my understanding of this whole situation. I will first point out though that revenge was Kayo's primary goal, NOT saving Enbizaka. So it's not like she's heroic, she still brutally murdered three innocent women/girls.

To correct one misconception here, HERs can go to the Heavenly Yard. It is ONLY demonic contractors who can't cross over. (And everyone goes to the Hellish Yard, that's where the passage to the Heavenly Yard is.)

I don't know if it's specifically explained why demon contractors can't cross over to the Heavenly Yard, but I suspect it's less about them not "deserving" to go to heaven and more that being a contractor makes their souls a certain quality that the Heavenly Yard by nature rejects. Remember, these yards are constructs with their own in-story mechanics, not all-knowing arbiters of who is worthy and who isn't.

(As an aside, living people also can't go to the Heavenly Yard, so I wondered if they might not be able to because the demon's regenerative powers makes it so they don't count as "dead" even when they're spirits? But that's just speculation.)

Kayo and Gallerian are just weird cases where they count as contractors on a technicality. Kayo was descended from a literal demon and so counts due to the soul-genetics of the series, and Gallerian supplied the "greed" himself while making a "contract" with someone who has demonic powers (Adam.)

I don't think contractors even necessarily had to go to Hell either, at least not before Gumillia came along, cause Allen says somewhere (SCaP?) that they also had the option of wandering the earth.

I think the "wander Earth" option only applied to Irregulars.