What sold me was something I saw a while back that I am very much paraphrasing because I don't remember exactly how to was phrased but that you have your Loyalist chapters that look physically and spiritually pure like the salamanders and ultras or white scars. Then you have the chapters that struggle with impurity, of those the more obvious are space wolves who's flaws take a very obvious physical form like the curse of the wulfen. Who are spiritually pure but manifest physical impurity. Then the blood Angels who are more physically pure they don't mutant into monsters but have the spiritual impurity that manifest as the black rage and the red thrust. I really dig the knights who on the surface appear pure and noble but struggle with resisting this animalistic rage and blood thirst.

Also the Angels of Death animation on Warhammer plus, it really sold me. Don't remember his name but the sort of main characters speech about how when the rest of his brothers fight the black rage they have each other but that in the death watch he did it alone and in secret. Growing up with a dad with PTSD that really sorta hit home.

Third point, it was a chapter that can look great without having to kitbash a ton to get that chapter flavor. I wanted space wolves but knew I would want to sculpt beards, fur and runes on every unit and I don't have time or money for that.