We report results from a test particle simulation to reveal that electron scattering driven by lower band whistler chorus waves propagating along a magnetic field line plays an important role to produce the butterfly distribution of relativistic electrons. The results show that two nonlinear scattering processes, which are the phase trapping and the dislocation process, contribute to the formation of the butterfly distribution within a minute. We confirm that the quasilinear diffusion estimated from the whistler chorus waves are too slow to reproduce the butterfly distribution within a minute. The simulation results also show that there is the upper limit of rapid electron acceleration. We expect that the upper limit of the rapid flux enhancement is an evidence that the phase trapping process contributes to relativistic electron acceleration in the heart of the outer radiation belt. Plain Language Summary Radiation belt electrons have various pitch angle distributions in response to global/local processes arising in the magnetosphere. Butterfly pitch angle distribution is a characteristic feature of the electron pitch angle distribution, which has the maximum flux intensity at a pitch angle lower than 90°. Wave‐particle interactions have been proposed as a driver for the butterfly distribution in the heart of the radiation belt. However, it is in debate how the wave‐particle interactions contribute to the formation of the butterfly distribution of multi‐megaelectron (MeV) volt electrons that is “killer electrons.” In this Letter, we report that lower band whistler chorus waves play an important role for the electron butterfly distribution at MeV energies. A numerical simulation was carried out and showed that electrons nonlinearly scattered by the whistler chorus waves produce the butterfly distribution at MeV energies. The simulation also showed the upper limit of the rapid electron acceleration in the formation of the butterfly distribution. The simulation results advance our understanding of a formation mechanism of MeV electron butterfly distribution driven by whistler chorus waves. Key Points A test particle simulation of electron scattering induced by lower band whistler chorus waves along a magnetic field line is carried out The electron scattering with nonlinear properties produces butterfly distribution of relativistic electrons within a minute The upper limit of electron acceleration for the butterfly distribution appears due to the upper limit of the nonlinear scattering