SPOILER for series 2
On the roof. When JM says "You're me." He seems genuinely happy. Pleased even.
It's not just about winning. If Sherlock's like him, then like JM, he too would be too bored to live. So making him die is a double score -- a gift -- as well as forcing Sherlock to dance one last time to JM's tune.
Might be a triple score even, if you count it as "I can't live without you, why should I make you live without me?"
How creepy is that? Oh, pretty damned creepy.
And here I thought that love suicides had thankfully gone out of fashion long ago. I don't think it counts as a love suicide when you force someone else to die, something which I think JM would ignore.
You know, I think I was happier when I hadn't had this particular thought...