It's gonna be a lot more painful to watch someone you know falling to the Dark Side, than to try to save someone you never knew.
I think it's pretty clear that what Luke saw was so bad that all he could think of was stopping it, even if he had to kill Ben to do it.
I'm thinking you must have seen some kind of alternate cut somewhere, because that's not how things played out in the version I saw. In the one I saw Luke had already stopped himself before Ben saw him.
So in your version he dis-engaged his light saber did he? He said ‘oh what am I doing?’ And then Ben woke up? If that was the case why did he push him through a wall?