The IRS treats payroll tax delinquency more aggressively than any other type. The trust fund portion is money withheld from employees. In the eyes of the IRS, you were holding their money and spent it instead.
Under IRC Section 6672, the IRS can assess the Trust Fund Recovery Penalty against any responsible person who willfully failed to pay over. This pierces the corporate veil. LLC, S-Corp, C-Corp, it does not matter. Talk to us before the revenue officer talks to you.