I love the film but even I am a bit miffed with how ridiculous Silva's plan is. That said you could say the same about most Bond films, hell even Casino Royale makes little sense if you think about it and is built upon a house of cards of contrivences.
It is clear that Silva is not thinking logically, or even when he is his logic is tempered by serious issues. He hates M, yet he clearly loves her on some level, and part of it seems to be about proving to her that she was wrong to abandon him. Look at his actions during the finale. He sends a helicopter gunship to macerate the house, yet gives clear instructions to his men that they aren't to kill M, when he finally catches up to her he shows tender concern because she's been shot, and even though he wants her to die he wants her to kill him at the same time.
He's clearly bonkers, he's just also fiendishly clever with it. I don't think it's that unusual a combination in a fictional villain.