The Chainsaw Majini does not actually decapitate the player characters, their heads remain attached similar to the censored/Japanese versions of Resident Evil 4. The player is only given the illusion of decapitation by the camera angle tilting and panning away from showing anything above shoulders upon death.
In an early trailers and screenshots of Resident Evil 5, the model of the Chainsaw Majini was implied to also serve the role of the Executioner Majini. The Chainsaw Majini was seen holding the axe.
Instead of screaming, they sometimes laugh hysterically before attacking with their chainsaws. The same laugh is used as the sound when viewing the Chainsaw Majini figure, followed by the roar of their chainsaw.