The only one I've regretted killing is Dusk and I'll never kill her again. And I only killed her that time to see what drop I'd get. She said something like, "My hero why have you killed me?" as she was dying and I still feel terrible for it. it will never happen again.
Spoiler for HIT-GIRL CORRIDOR *BEST.SCENE.EVER!* :
Did you have a couple drinks to forget that terrible act you did.
I kinda regretted killing Priscilla since she didnt wanna fight anyway. And also those mushroom things that didnt attack me at Darkroot but later changed my mind when they were bitches at the Great Hollow.
First time I met those cats I literally shit myself. I just killed Sif and was feeling good when I saw something huge run by. I was getting destroyed until I stood behind some trees. Everytime theyd attack I went for cover and it was cheap but saved my ass.
What the hell are dung pies good for lol. I dont think ive ever tried to use one at all.