if obi-wan were nicer to anakin then anakin might not have turned to the dark side, he probably didnt want to be around some1 like that nemore...you see how he had no problems killing obi-wan, but he wouldnt kill his son, thats because luke didnt treat him like he was nothing, well, luke tried to kill him, but thats besides the point, luke didnt go around telling him, dont do this, dont do that that way, do this this way, obi-wan always had to have evrything his way, so anakin was sick of it, and when they told him that they he could keep his life if he turned to the dark side he jumped at the chance, probably because of the way that obi-wan was always treating him...
If Obi-Wan had been 'nicer' to Anakin he might have gone off the rails sooner. A relationship like that demands discipline and Anakin is absolutely FULL of himself. His head would have been the size of Coruscant of Obi-Wan had not kept him cut down to size.
Vader was perfectly happy to kill Luke if need be! It was Luke's helplessness and heartfelt appeal that persuaded Vader, not any innate niceness Luke had shown to him! After all, as we know, Obi-Wan once thought as Luke did.
Obi-wan is stern with Anakin because he is his student. But he wasn't always that way with him. He joked and giggled with Anakin in the elivator at the beginning of EP II. Obi-wan cracked dry jokes with Anikin the entire movie. The two were friends.
And as far as Obi-wan being at fault for Qui-gons death. . . . that's the dumbest thing I've ever herd.