Soichiro's death. He does die happy, but the logic around the whole situation is just horrible. He dies trying to catch an infamous serial killer. That killer, not entirely beknownst to him, is his own son. While he's dying, since he has shinigami eyes, he notices he can read Light's lifespan and, relieved, says he's glad it means he can't be Kira, which he is... And the last thing Light tries to do before he dies is try to make him give him Mello's name...

... Must I really mention L's death. ;~;

Light's death is incredibly sad to this troper, despite his status as a Magnificent Bastard ...

Mikami's backstory would've made him look like The Woobie if it weren't for... the way he turns out. Which in fact makes it all the worse; the change he goes through is simply tragic.

And Mello's.