Don't assume you know everything, there are a number of surprises coming in Londo's and G'Kar's arcs.
JMS doesn't mind showing you the ending, considers the "how" you got there at least as important as where you go
A Prequel TV Movie In the Beginning
, contains some spoilers up through S4, but, he has often recommended it as a Starting point, instead of The Gathering
You've known for a long time that G'Kar kills Londo, this is why Londo has such an aggresive relationship with G'Kar, even over and above G'Kar being a Narn. Londo has always known, from the first time he saw G'Kar's face that G'Kar was the man of his dreams.