Vader did it to save his son, not to “kill the bad guy”. There’s a big difference there.
But that just makes Vader’s motives more selfish. Palpatine was slaughtering people by the shipload (not to mention subjugating an entire galaxy). You’d think Luke would be justified in taking this guy down. But no, all Luke could do was make sure he stayed Zen. Great if you’re scoring brownie points at a Buddhist retreat maybe, but not much help with the war effort.
Vader’s position was purely selfish. “Hey, no-one messes with my kid!”. He was merely protecting his own. How does this earn him instant Jedi nirvana?
Therein lies my problem with Vader’s “redemption”. Even if you can make the case that a mass murderer can absolve himself with one good act, Vader’s one good act was hardly selfless.