Been years so I don't completely remember how it worked but for Metro 2033 in addition to finding enough positive karma spots (The light glow and sound effect.) while not losing these such as from killing human enemies and other conditions, major losses such as disobeying Khans directions being indicated by a dark glow and deeper sound effect but for small losses such as kills there's no indication at all and then having X amount of karma points for the decision at the end of the game.
For 2033 there's also Hunter and during a particular segment ignoring him else even with enough karma you can't avoid getting the bad ending though both 2033 and Last Light primarily just have a small change at the very end so it's easy enough to just view the alternate ending afterwards.
Do not shoot the Dark One during the hallucination sequence.
Unsure if Redux is the same but 2033 and stealth could be a bit hit & miss so sneaking away from or knocking out unaware enemies could be a bit difficult and then for a few missions the kill restriction is lifted entirely though it's never really cleared to the player but if combat seems unavoidable it's probably one of these situations.
(Redux did remove the sneak suit you could upgrade to at one of the train stations and also the heavy armor you could buy a bit later.)
The vagueness and various arbitrary conditions (Looking at stuff for a set amount of time, interacting with objects or just avoiding certain things.) plus the silent but still counted for loss of smaller points of karma can be pretty difficult, don't remember exactly how many points you needed either plus the final condition or it all amounts to nothing anyway if you don't do that properly.
Last Light relaxed things a bit and Exodus is even more forgiving though both retain the secondary condition for getting the better ending even if you otherwise have the karma criteria down.
Guessing CE and getting flag properties or what scripting goes on could also be a way to force this into a specific outcome, it's kinda vague so there's not much info on the karma system other than what gives points and then testing also revealing the loss system even without the whole audio visual cue.