I loaned money to a developer who went bankrupt. I was supposed to get interest on the loan. Can I deduct the principal plus interest?

You can deduct the unpaid principal if you can show that the debt is fully worthless. The fact that the borrower filed for bankruptcy does not automatically establish worthlessness. However, even if you can establish worthlessness, you can only deduct the unpaid principal. As a cash basis taxpayer, the interest you were to receive was never reported as income so no bad debt deduction is allowed for this amount.