First is that she succumbed to her injuries and punishment of the previous day, meaning the soldier's statement was truthful. Second is that she committed suicide somehow (by herself), maybe to spare Byleth the pain of having to execute her herself? Third is that she was forced/convinced by Seteth to commit suicide or was killed as humanely as possible (as opposed to decapitation)? Sadge, all she wanted was to have her Teacher be the last thing she saw and heard. I suppose Seteth could have spared her maybe? Though that's copium at this point. Church bastards! I will never forgive you!
Pages 59 to 64 fooled me into thinking that she might have been spared—a realistic story, but quite sad.
Like one commentator in the Chinese translation pointed out (thank god google translator exists): in the second last pic, both have a halo, so does this mean Byleth took her own life?