He'd spent nearly three years hiding in the shadows, and it had become boring. Even as a child, he'd been able to kill to keep his mind off the boredom and to punish those people he felt like punishing. But all this time, he hadn't been able to kill anyone or do anything to stop the breakdown of his criminal network; he hadn't wanted Sherlock to notice him before he was ready.
Because he had big plans in store. And so he contented himself with preparing for his big reveal.
He hadn't yet decided when to show his message. He wanted to shock the world - but most particularly Sherlock - as much as possible. But all that went down the drain when he heard of what Sherlock had done.
"Oh, Sherlock. You naughty boy," he had grinned. His enemy had dropped from his place with the angels, and his brother was dealing out consequences. It was wonderful, but also possibly deadly - if he allowed Sherlock to die, then he'd become more bored than he had ever been. He couldn't allow that to happen.
So he pressed the button, and let his message ring out across the world.
Did you miss me?
Of course they did; even ordinary people got bored, and the end of Charles Augustus Magnussen signalled a hell of a lot of boredom thanks to the information that died with him.
He did loathe that man. All that power at his fingertips, and what does he do? He blackmails his way to the top of the chain. No understanding of the honour of the underdog. It was despicable.
As was what Moriarty planned to do.