Why do people assume Silva would plan the sequence of events every step of the way when it seemed like his organisation was much more powerful/wealthy than it first appeared and he had multiple plans put in place throughout the world to service his insanely egotistical plan of hugely humiliating M in the media spotlight before killing her?
Because we watched the movie. And it was his plan to get captured. It's not an assumption.
And besides, it would've been much easier to kill her in her house. The humiliation of MI6 could've still happened--and in fact it WAS happening with the release of the agents names and their deaths... and then Silva could've walked into her house, woke her up, and then put two bullets into her brain.