no, I mean use death magic to cause necrosis
I could go yes, MAGIC! Of course it works. But your asking for some realism I suppose. So a more long winded reply.
Depending on where the arm was cut, Necrotizing the arm would not stop the bleeding, let alone the issues with shock trauma from the arm being lost. The closer to the shoulder the worse the issue is.
Necrosis is the death of tissue, usually from the loss of fresh blood being pumped into that area of the body. In the "Real world" necrosis is caused by the stoppage of blood for to long, disease, injuries or infections. It is why if you use a tourniquet for an injury/medical procedure or if someone has certain health conditions you are not suppose to have it on to long, if at all.
Gangrene poisons the rest of the body extremely quickly, to the issue it will necrotize more of the body from its spread if it is not caught fast enough.
There is also a difference in the type of gangrene Wet or Dry. The Wet type of gangrene is caused by bacteria, and the Dry type is just dead tissue caused by something else. However Dry Gangrene can easily become Wet as bodies are constantly being attacked by bacteria that the body fights off and that area is weak and will have next to no defenses against harmful bacteria.
So
yes, you could say the spell causes the stoppage of blood and the acceleration of Necrosis. However, when large areas of tissue die due to a lack of blood supply, the condition is called gangrene and that spreads fairly rapidly if untreated. If the body is constantly exerting itself, guess what spreads even faster!
TL/DR Yes! MAGIC!
