TvTropes has upvoted MCU!Mysterio as a CM. I'm wondering if you agree with the assertion that is being made to justify him qualifying since you noted that in your opinion, he did genuinely care for Peter and did regret to kill him.
Also as a side note, they've upvoted Ardos from the Pokemon Orre games too, through I'd disagree with this upvoting since I agree with you that he needed a third game to qualify and dosen't do enough in the time that was alloted to him.
Ardos has been added here too, as he was already a YMMV example, so I figured why not?
On giving it second thought, Mysterio did have fondness and pity for Peter...but he didn't care about him enough to not just shrug it off and try to kill him or (alternatively) ruin his life without a shred of remorse, hesitation, or desire to find any alternative. He found it regrettable that he had to kill the boy, but he didn't find it not worth the effort to do so as long as his selfish plans were serviced by Peter and his friends getting wiped out. Kind of like Ego and how he shrugged off his love for Meredith Quill like it was nothing despite saying that it hurt him to give her that fatal brain tumor.
So yeah, Mysterio is a valid qualifier after all.