"委托金亏空者" can be translated as "the person responsible for the deficit of entrusted funds" or more concisely as "the defaulter of entrusted funds" (though "defaulter" might slightly imply a failure to meet an obligation rather than strictly a deficit, it's contextually close).
A more precise and natural-sounding translation might be "the individual who caused the depletion of entrusted funds" or simply "the one who misappropriated/caused a shortfall in the entrusted funds," depending on the specific context and the nuance you want to convey.
For a general and straightforward term, "the person responsible for the shortfall in entrusted funds" would work well.