Heath Ledger vs 2Pac vs Notorious B.I.G - a parallel?

The Dark Knight has been smashing box office records ever since it was released late last month. Cinema goers and critics worldwide are raving about Heath Ledger's performance as the sadistic Joker, although his on screen sadism has not been reflected on screen like in SAW, the character's dark side is very evident throughout the movie. Chilling, dark, think of any adjective to describe his performance, you will find yourself a few words short at the end of it all. The subject of this post however is not about HL's performance. It's about his movie minting millions after his death. Consider the case of gangsta rap icons Tupac Shakur and Notorious B.I.G. Both of them sold tons of records when they were alive. Their murders however changed the equation. Their music is still popular and is making more money than when they were alive. Heath Ledger's case seems to be identical. Is it just me or do I see a parallel here? A conspiracy theory? 2Pac and Notorious were bumped off and their posthumous records have sold millions of copies. Was HL murdered? Or did he overdose on prescription pills?