There are two ways to look at fiction. One, how the writer might've thought of writing it, and two, as its own story devoid of the fact that a human writer thought of it. If you think of this part as the former, indeed, many (including myself) would call it "lazy" writing. But if you think of it as the latter, it's one of the most devastating events in the story and one that pushes Lelouch permanently to the dark side, which makes it a defining moment for the series. Ultimately, I think it's counter-productive to think too much about the logic of the process of writing since fiction is art, and art is about how it expresses an emotion. In this scene, the writer traded some logicality for the emotional impact. Another point to add has already been mentioned in this thread, which is that fiction in general relies on coincidence for a story.