Although the Jean-becomes-Phoenix story had a slow build, her actual death happens quickly. A lot has been written about X-Men #134-137, in which Jean become Dark Phoenix and ultimately kills herself because her own power is too much for her to contain. I’m not going to say much more about that arc—it’s one of the most written about stories in Marvel history. But look at that one sequence, above.
Jean knows she has to die, she knows her own power is too strong for her to contain. She doesn’t want to have live and be in constant vigilance against the power inside of her. So, she says goodbye to Scott. Scott sees what she’s going to do and cries out. And in that moment, she has doubt—she screams his name and puts her hand reflexively forward in a defensive position. She’s not reaching for him, because the palm is up—she’s telling him at once to stay back but also crying for him. At the moment of her selfless suicide, she realizes she doesn’t want to die.
And I’m sure that’s all John Byrne.
Yeah, it’s the most written-about arc in comics because it’s one of the best.