Lenin called this kind of individualistic, unorganized violence "revolutionary adventurism", and strongly condemned it. The lesson is not that violence isn't effective, it's that unorganized violence isn't effective. Sufficiently organized violence can be very effective indeed.

That said, the same is true of nonviolence.

Well what other tools do we have? Waiting for the market to fix things is also destructive and harms orders of magnitude more people than violent direct action does; democracy is wildly ineffective compared to violence even at its most optimistic; what else remains? Fleeing the planet?