Yes, there are many ways in which a healing effect"healing effect" could be used to kill the target. It's the way in which your healing effect can be directed using your magicworks which would be key. So if the effect can be directed this would help.
Auto-Immune diseases
If the healing effect could be linked to making the immune system better you could get it becoming over active this can lead to conditions like MS which could be used in an accelerated fashion to kill, this might be a bit slow.
Cancer like
The otherAnother option is if you can accelerate the actually process of splitting the cells within the body. Then you could have various forms of cancer in this way, just heal the body till it creates a huge mass of cells. Which is now growing uncontrolled in the target, this replicates cancerous growths.
Bacteria
If you can accelerate cell splitting then you could also heal not the person but the bacteria that is already present in the target, we have many many different type of bacteria or fungus in us and an imbalance can cause many issues, a mild condition is Thrush but there are other bactericabacteria or fungus that can also be fatal, or cause other condition such as blood poisoning.