I can understand kirk
's complaints about the story having been (somewhat) spoiled. Frankly, I don't think anything was gained by revealing Londo's fate as early as season 3 (I would speculate that JMS did it because he was afraid of cancelation leaving alot of unresolved threads).
However, Londo's fate is quite logical. He flirted with the Shadows when it suited him and, when it no longer did, he decided to end the relationship. With nuclear weapons. His comeuppance was inevitable (a fact that Morden
made quite clear). The fact that the Drakh resorted to those very same nuclear bombs to coerce Londo only reinforces the karmic aspect of it. Also, had the Centauri not been so agressive recently (something that was only possible due to Londo's actions) the other nations might have been more lenient, deciding not to resort to orbital bombardment. Again, another layer of karmic tragedy.
Londo's sacrifice is also quite logical. Some have said that season 1 Londo would not have done that. I'm not so sure. Londo was always a patriot. Everything he did was for the greater good of his people. In this instance he was just being himself, I believe.
Nothing about it was arbitrary or stupid.