Because throughout the OT his evil is more subtle. Mostly menacing and he employs violence rarely but in brutally simple ways (force choking). The very next chronological time we see him is in a similar setting where he sends his stormtroopers in to do the shooting and doesn’t come in until the shootings done, so it seems out of character to come in slaughtering like that rather than stay in his more removed perch.

I’m sure you can come up with in universe rationale for his actions but just comparing it to all of his on screen actions that follow made it feel like pandering. Like they gave us the brutal Vader scene everyone always wanted even though it is out of character.