As you remember, Kendall is broken after Season 1 finale. He wanted to gain some kind of control and power in his life.
i think it was a display of kendall feeling absolutely powerless. he didn’t want to shut down vaulter, he did it “because his dad told him to.” he’s firmly under his dad’s thumb and hates it. he stole the batteries so he could feel some semblance of control in his life again, some modicum of power, however small.
reddit comments, u/justsomehandsomeguy