My builder stole a large amount of money from my construction account and then killed himself. Can I claim the amount that was taken as a theft loss?

for 2018-2025 personal casualty losses due to theft of funds are not deductible unless the taxpayer has a personal casualty gain, to the extent of such gain. You would need to speak to a lawyer to determine if a claim can be filed against his business or estate.  

For tax years 2018 through 2025, if you are an individual, casualty and theft losses of personal-use property are deductible only if the losses are attributable to a federally declared disaster (federal casualty loss).

https://www.irs.gov/pub/irs-pdf/p547.pdf