My take on this is Mashima wasted our times now with the Hien story (and honestly the Raizen one, for a short as that was) if he was just going to unceremoniously off him. I'm not opposed to character deaths but it's got to mean something in the grand scheme of things. Like Frey's Sister probably should've remained a factor for Frey's character arc, but it seemed wrapped up far too quickly for shock value. Now I'm expecting none of these characters to survive, and I feel I'm wasting time learning about them.
This is why plot armor for heroes and villains are a thing in stories, so you can build something to look forward to. This is starting to feel like another mini series from Mashima now.
Mashima is way better than this; Rave Master, Fairy Tail (especially), and even Edens Zero show he can build up characters extremely well and give even a side character an great mini arc and fight.