I think it’s really good writing actually. It doesn’t come out of nowhere as a lot of people seem to think, look at episode 13 for example when Ash and Arthur are fighting. Ash clearly has the upper hand until Eiji yells for him, this is when Arthur manages to land a pretty significant blow. Later in the series, Sing mentions that Ash becomes a docile cat when he is around Eiji.

We see over the course of the show that Ash becomes more and more comfortable in being emotionally vulnerable when It comes to Eiji, and this of course translates into physical vulnerability, ultimately what leads to what happens in ep. 24.

The Snows of Kilimanjaro and Perfect Day for Banana fish both end in the death of their main characters, so it is pretty strongly foreshadowed. Particularly when it comes to Ash, he knows, even from that very early Halloween scene that he “cannot mingle with normal people” anymore.

It is beautifully written. Ash gets to go on his own terms, in the anime he himself says that Lao misses any vital organs. At least in my interpretation, Ash has come to terms with the fact that the only way he can protect Eiji from his life of pain and danger is by letting him go. The only way he can atone for the guilt and pain of his own experience is to let go. Do I agree with what he did? Hell no, I wish he and Eiji could’ve had a proper goodbye at least. I think what is written is even more powerful than that. It is a beautiful physical demonstration of the vulnerability Ash has achieved in allowing himself to love and be loved by Eiji. The smile on his face as he passes on says it all.