Worst..

Gotta say Marnie is one of my favourites, a far more psychological take than Hitchcock's usual fare. Rutland's (Sean Connery) obsession with 'fixing' Marnie is explained, but it's left up to the viewer to deduce rather than being spelled out, which gives a much more natural feel to proceedings but runs the risk of viewers being left in the dark if they don't put the pieces together.
It is established fairly early in the movie that Rutland was an expert in Zoology and fascinated with animal psychology, particularly female psychology, and also psychology of the criminal mind. He boasts about training a Jaguar to trust him, this is like a hobby to him. Though he was captivated by her beauty, it was more her mind that got him obsessed. Could he apply what he knew to this obviously troubled woman? He is himself a flawed character, with an unhealthy desire to 'fix' things or show dominance over others by essentially bringing them to heel.
There is no healthy connection, even at the end Marnie will not suddenly be ok now that the truth is out, but that is the nature of this relationship warts and all. This is not meant to be a romance.

In no way is it his worst

He made like 50 films. I doubt anyone's seen them all, especially his British period. Some consider this his last great film (watch on Youtube the hour long Making Of documentary). I didn't hate it but I doubt that I'll watch it again any time soon.