What if...
Vader and the 501st are marching up on the temple when Vader sees two silhouettes in front of the entrance. He activates his light-saber, its blue blade a highlight in the dark, as he prepares to slaughter what he saw as jedi. The two figures emerge into the emitting blue-light and that's when Vader stops in his tracks in disbelief. Standing in front of him are Padme and Shmi Skywalker (pretend she didn't die in AotC).
"Oh Ani, my son", says Shmi
Padme looks into her lover's eyes as tears swell in her own.
"Mom, Padme", says Vader with weakness in his voice.
Vader halts his troops and runs towards the two people who had ever brought him true happiness in his life.
You finish: what happens next?
Bonus round: make one of Rebels/ANH era Vader. What would he do?