Darth Vader was incapable of weakness or exhaustion. The Dark Side had lifted him above such things—he believed. He had told himself he believed.
He didn't know. He had changed again; it didn't matter any more. Nothing mattered, except his son's agonizing death, and his daughter about to be captured on the moon below.
Anakin knew Palpatine's lightning would kill him. Separate him from the last of his family. Grant death the final victory over him, and possibly accomplish nothing. He didn't care. He was tired, and the Skywalkers had had enough—enough of masters, enough of suffering at the hands of their masters, enough grudging submission.
After everything, there was still something he could do for his family. He would keep his children free, or die trying.