= Solution
\b[The central carbon is approximately $sp$ hybridized, with two opposite sigma bonds and no carbon lone pair; the electron-rich donor ends are sulfur and nitrogen.] The 16 valence electrons allow the principal <resonance structures>
$$
\mathrm{^-S-C\equiv N\ \leftrightarrow\ S=C=N^-}.
$$
Each has octets and a linear S–C–N skeleton. Carbon's two perpendicular p orbitals support the pi interactions. The first contributor emphasizes the C≡N bond and sulfur-centred negative charge; the second distributes pi bonding over S=C=N and puts negative charge on N. The real anion has partial multiple-bond character and delocalized charge rather than alternating between these drawings.
Nitrogen's greater <Allen electronegativity> favours electron accumulation there, while sulfur supports a diffuse, polarizable electron-rich region. Thus \b[both terminal atoms are important electron-rich sites relative to central carbon]. “Greatest electron density” should not be interpreted as a literal comparison of all core electrons on different elements; the chemically relevant comparison is valence charge, donor density or electrostatic potential. The exact partition of negative charge between S and N is not uniquely determined by electronegativities or Lewis structures alone.
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