It depends.
If you notice their problems and have the opportunity to help them, you should definitely do it.
Keep in mind however, that at the end of the day, they are responsible for their own well-being.

It depends on the specific situation. I’m only one person.

Its not nice, but at the end of the day we are accountable for our own existence.

Seems like there's a lot of room for gray area here, but if you saw someone in a life threatening situation but you had zero intention of helping them in any way (including calling for help, looking/thinking of ways to help), that's pretty dang bad. If you were too afraid to help because it would've also put you in a life threatening situation, that's a little more understandable. Best case scenario, you at least called for help and tried to come up with SOME solution while you waited for help to arrive (even if they ended up dying)

It's worse, quicker death is a mercy.

Philosophically/analytically I want to say yes, but then I wouldn't feel nearly as guilty if I let someone I disliked die as I would if I'd killed them, and I wouldn't expect/think it's fair to receive nearly the same punishment if any. I guess on an emotional/spiritual/karmic/whatever level not helping someone to survive and killing someone are very different things, even if they seem functionally the same.

It's negligence, which can be criminally charged depending on the case details, afaik that's next to but not up there with murder.

In these situations, circumstances and context matter - while I do think that we all have a minimum moral obligation to help each other, I do not believe you can really blame someone for not taking action in all cases - sometimes our own fear and survival instincts kick in, sometimes we might lack an ability or skill needed (like jumping in after a drowning child when you can't swim - then you just end up with two dead instead of one)

Nearly, yes. Imagine for example a guy standing near a pond where a three year old kid is about to drown and he is doing nothing to help.

That's kind of a gray area for me. There was a high profile case here recently where some girl encouraged her suicidal boyfriend to end his life, which he did. She was charged with it as a crime, but it was a very contentious. IMO she should have in that particular case, but someone who does nothing to help shouldn't be found guilty of anything short of "being a dick".

Our sympathies and abilities and loyalty is limited merely to our social group. We have exceptions though we cannot save the world. So you might feel bad, but after you spent all your ressources you end up as a victim yourself unable to survive. Difficult.