  We revisit the phenomenon of instability of solitons in the generalized
Korteweg-de Vries equation, $u_t + \partial_x(u_{xx} + u^p) = 0$. It is known
that solitons are unstable for nonlinearities $p \geq 5$, with the critical
power $p=5$ being the most challenging case to handle. The critical case was
proved by Martel-Merle in [11], where the authors crucially relied on the
pointwise decay estimates of the linear KdV flow. In this paper, we show
simplified approaches to obtain the instability of solitons via truncation and
monotonicity, which can be also useful for other KdV-type equations.
