Luke should have talked to Leia about young Ben instead of killing him in his sleep

No, no way, would never fit in with KK's grand scheme, to keep the Original Trio separated at all costs. Plus, Luke was meant to be a hermit at that point.

Why do people never tire of demonstrating how retarded their perception of this movie is?

Honestly, there were a million better ways to do it; but my guess would be Rian wanted to do flashbacks because of Rashomon (I don't credit him with enough literacy to know that it was originally a short story by Ryunosuke Akutagawa), and that was the best his writing skills could turn out.

Luke should simply have sensed that there was still good in him and refused to take down his relative - as per the established character of Luke Skywalker.

In fact, given that he didn't, and given that TROS redeems Kylo (as far as I've read anyway), this makes this particular aspect of TLJ even worse...