There's an alternative ending in Metro 2033, which unlocks when you're a "Good Person" by game's definition. This involves talking and listening to people, interacting with objects and other things. Each of such actions triggers a Moral Point.
Now does killing humans cause you to lose Moral Points? Do you need to purposefully avoid killing them?
There are some choices regarding the killing of people which will affect which ending you get according to the guide I used they are the following this answer will only contain action needed to do in regard to not killing or killing characters for the full guide look here:
2/Bridge: The box of childrens' toys in the traincar awards morality. The traincar is on the first section of the bridge. Remember, enemies are reduced to a trickle (if not completely eliminated) if you decide to kill everything before moving on.
4/Frontline : Related to the Invisible Man(1) achievement. Completing this whole level without killing anyone will increase your level of empathy. Note this probably conflicts with the Rescue Ranger(2) achievement, unless you've found some means of tranquilizing enemies like that augmented wild man Adam Jensen in Deus Ex.
4/Black Station: Successfully eavesdrop on the first two Nazi sentries to sway your morality meter towards good. Use a stealth approach for this. Killing these enemies with loud weapons alerts the whole base.
4/Black Station: Related to the Merciful(3) achievement. Completing this level without killing anyone will mark you as a good guy.
down here are the achievements referred to in the quoted piece:
(1) Invisible Man is an achievement/trophy in Metro 2033 and Metro 2033 Redux, awarded for not killing a single enemy on the level Front Line. Front Line is a very large level with many enemies but with enough restraint and caution, players can avoid all combat.
(2) Rescue Ranger is an achievement/trophy in Metro 2033 and Metro 2033 Redux , awarded for rescuing the Red prisoners from the Nazis in Frontline. The prisoners are found near the end of the level in a Nazi camp. Going for this achievement/trophy will render Hedge-hopper unobtainable as after the prisoners are freed they will be unkillable thus the other achievements have to be done on other separate playthroughs.
(3) Merciful is an achievement/trophy in Metro 2033 and Metro 2033 Redux, awarded for not killing or knocking out a single Nazi soldier on the level Black Station. This is quite a bit easier said than done as any Nazi will immediately fire upon the player if detected
