It’s a long posed ethics dilemma, usually based on an emergency room with a doctor doing triage on some number of patients with some varying number of serious and minor injuries.
Fast forwarding through all the discussions: yes, you would be a killer. The moral permissibility of the action doesn’t negate the nature of the action. You could potentially be a lifesaver, depending on the context of the killing and your intent.
No, it’s not always wrong to be a killer, or to sacrifice someone to save others.
Yes, it almost always is wrong though.
No, it’s not realistically possible for someone making that decision to know the caveats that might make the sacrifice justifiable.
In general the practical ethical action is to prioritize the “sure thing”, and otherwise direct your efforts where they can do the most good in a situation where there’s limited time of resources to treat everyone.
The guy coming in for a physical is nearly certain to survive, so he should be told to leave and promptly ignored until he stops being healthy.
The unresponsive guy with a concave dent in the middle of his chest, not breathing, and a weak irregular pulse has pretty low odds, so you make sure his head is positioned well if he starts breathing and move on.
The person with a bubbly chest wound and wet bloody cough is probably able to be saved if you help them.
Sacrificing people who would have lived just keeps those people away from the hospital, so it does more net damage and costs lives, from a strictly utilitarian perspective that ignores “bodily autonomy” and the like.