By a theorem of Suslin, a Tor-unital (not necessarily unital) ring satisfies excision in algebraic K-theory. We give a new and direct proof of Suslin's result based on an exact sequence of categories of perfect modules. In fact, we prove a more general descent result for a pullback square of ring spectra and any localizing invariant. Our descent theorem contains not only Suslin's result, but also Nisnevich descent of algebraic K-theory for affine schemes as special cases. Moreover, the role of the Tor-unitality condition becomes very transparent.