Spoiler How do I die in the end, I only killed the people who needed to be killed, and I didn't kill any innocent. Maybe it's because I shot first at enemies too much and didn't wait for them to attack me? Off-topic: I'm excited for the DLC this month, I hope it's just a off-story DLC, so you go to main menu and you just play from there, and it's not connected to the main game in any way.
Not killing is not the only thing that gives you karma-points.. you need to find hidden areas, listen to certain conversations etc etc..
Take your time and explore every nook and cranny and listen to every conversation to its full length. The two choiced killings might make a bigger difference, but they're not essential.
I know how you feel, i got the bad ending too and i did: Listen to most of the conversations, Just killed few guys throughout the game, always played instrument when i see it, i always gave a bullet to a homeless guy, i gave back the lost teddy bear to the boy, i killed the watchers on the Bear Monster's back and many other things... Still got a BAD ENDING. However, i just looked on metro wiki about moral points and, oh my god, there is a dozens of ways to earn moral points that i probably skipped. I'm still pissed thought. (And i also didn't killed Pavel nor Latsitsky)
Well, if anyone would like to see the "Good" ending, I've had it up on my channel for awhile... http://youtu.be/05dbbwtKnsY
I think it's considered "bad" because of the following: 1. The Rangers are practically wiped out. The Red Line must have suffered a huge loss. There is speculation that the Reich might seize the moment to try to "purify" the Metro. I'm not sure what the Hansa has to say about that. The game doesn't show the same level of power the Hansa seem to have in the novel. 2. Could be just me, but I think it shows that the situation is back where it started. Humanity and the Dark Ones have gone their separate ways. Artyom's role as a Ranger was fulfilled, but not his role as the 'ambassador' (if that's appropriate to say?). With him dead, who knows when the next contact will be made?