Vader was only trying to get the plans that were stolen. All these people were in his way shooting at him, it’s either sit there and let the firing squad kill him, or mow them down with a lightsaber. I myself, in that situation would have done the same thing.
Because if he loses the plans, he has to face the Emperor and the consequences for failing.
Darth Vader’s a fictional character though, justifying a story decision with in-universe facts doesn’t work. The writers could have just not had that be the case.
Imagine if a WW2 movie, which people like to reference to RO, ended with a scene in which a Gestapo agent slaughtered a bunch of resistance fighters. And then people cheered.