This more going to be fluff than anything. I remember prior to the Ringed City's release that some people thought the dragon boss was going to be Kaathe, who had somehow turned himself into a dragon, and honestly, I kind of wished they gone that route because I feel like it would've tied up a few loose ends.
Basically I think it would've gone like this: From the beginning Kaathe has had no interest in Man and has wanted to become a Dragon, and as such assumed that without the Fire, the Age of Dragon's would return, hence why he wants you to usher in an Age of Dark in DS1.
But that doesn't work and in the events of DS3, now knows the Fire doesn't just need to be extinguished, but consumed to completely disappear, hence the Lord of Hollow's objectives. But Kaathe either looses patience with this plan or maybe realizes he can't become a true dragon, so he goes for the next best thing: The Abyss.
The Abyss warps men's humanity and transforms men into monstrosities of great power, so he assumes if he's consumes enough dark souls he'll become a sort of Abyss Dragon. As he consumes dark souls his influence warps the dark, much like how men can, suddenly it makes sense as to why Aldritch begins eating people, why the Locust men can't think past their own stomachs, the bugs that nibble at the flesh, and gives a chilling new meaning to "Fear not the Dark, my friend. And let the feast begin."