Yeah i completely agree he had to die, however it was how it was implemented. There was no build up, barely any character development to lead into it, just a quick removal of the face of TLOU to make room for a forced story, thats how it felt.
I believe Lev was the reason too, but its not a realistic outcome for Ellie. She literally saw Joel get tortured to death by her, she saw Abby kill her friends, and Ellie cared little for any one to the lead up to finding Abby.. than all of a sudden gives her mercy? I dont know, it just felt unnatural.
True that there was little build up, but i kinda like that tbh. I like these WTF just happened moments and then seeing the aftermath of that. A big reason why I really enjoyed the first 5 seasons of Game Of Thrones.
Abby gave Ellie mercy twice, so maybe ND thought Ellie killing Abby would make her seem like the more ruthless and cold one. I dont know. I would be fine with either outcome.