Nigeria's expanding public debt profile has become a central point of national policy debate, catalysed by the Tinubu administration's assertion that "borrowing is not leprosy." This paper advances a structured counter-argument: public debt is not inherently destabilising; rather, it is the mismanagement of debt-characterised by consumption-driven borrowing, chronic underperformance of capital expenditure, and a persistently narrow tax base-that generates fiscal pathology. Drawing on data from Nigeria's Debt Management Office (DMO), the Federal Ministry of Finance, Budget and National Planning (FMFBNP), the Central Bank of Nigeria (CBN), and multilateral institutions including the International Monetary Fund (IMF) and World Bank, this study analyses Nigeria's fiscal transformation between 2010 and 2025. Key findings reveal a structural divergence: total debt stock expanded by approximately 891% over the period while government retained revenue grew by only 258%. By 2024, Nigeria's debt service-to-revenue ratio stood at 69%-nearly double the 30-40% threshold recommended by the IMF and World Bank for developing economies. Capital expenditure execution in 2025 reached only 17.7% of allocation in the first three quarters. The paper proposes a threemetric diagnostic framework-comprising the Debt Service-to-Revenue Ratio, CAPEX Execution Rate, and Non-Oil Tax-to-GDP Ratio-as a more reliable indicator of fiscal sustainability than the conventional Debt-to-GDP ratio. The study concludes with evidence-based policy recommendations anchored on fiscal transparency, revenue resilience, and the golden rule of public borrowing: borrowed capital must generate returns commensurate with its cost.