I don't know. It seems to depend a lot on context really.
Also, effort.
Deliberately killing someone takes effort. You have to go out of your way to do it, so you probably would have some reason or motivation for doing so. (even if it's as disturbed as 'for the fun of it' or something similar.)
Similarly, if someone is going to die soon without intervention... That's also a matter of effort. You have to go out of your way to do something about it. The effort involved may vary considerably depending on the situation, but it doesn't change the basic fact that you will need to make some kind of effort, no matter how small, to prevent a death.
We therefore have a comparison between making a deliberate effort to cause a death, vs making a deliberate effort to prevent one.
That's obviously somewhat of a different situation. Lack of effort in either case leads to the opposite result. (If you don't put in the effort, the person you're trying to kill will survive.)
The other thing is unpredictability. - it's all well and good to pose the question 'if you could save someone', but in reality many times it's not that simple.
If I saw a drowning child, would I actually be able to save them in the first place?
How about a fire? The risk here is such that not only could I fail to save the person, I could die in the process.
Or perhaps something as simple as a person having a heart attack, seizure, or other medical problem.
Not being trained in medicine or even first aid, there's many medical emergencies that you can make worse by doing the wrong thing.
Moving an accident victim with certain kinds of injuries could make matters much worse for them. - in which case, good intentions would make the situation worse if you don't know what you're doing.
You might say, call an ambulance, or something. Which is of course a good point, but what you're doing there is not so much saving someone's life as delegating the responsibility to someone else... (granted, if you didn't go to get help...)
Anyway, it's all a lot more complicated than it sounds on the surface.
Going out of your way to deliberately harm someone else is a lot less complicated to understand than the unpredictable effects of not making any effort to try and save someone else.