They never said but I think it was because she nagged and demeaned him. Plus, didn't he have someone else he had something going on with (the long distance calls he made)?

Yes I forgot about Jeff watching him dial and noting it was long distance... Also, I think her belongings were picked up by a woman at the other end... Thank you for reminding me! Definitely needs a re-watch :)

Yes, he seemed to be cheating with the long distance calls and another woman shows up the same night of the murder. whenever we saw him with the wife they were fighting.

Cheers!

It doesn't matter because the murder was a MacGuffin.

What's a MacGuffin??

It's a kind of I guess you'd call it a plot device, except it doesn't really have anything to do with advancing the plot. In other words, we didn't have to know why he killed his wife, didn't take anything away from following the movie. :-)

Ok cheers, still I was just curious as to why and as mentioned above, there's at least a couple of clues... Thank you!

MacGuffin was a term coined by Hitchcock and/or somebody else along with him (I don't recall who.) It's a plot device that's very important to the characters, but "the audience don't care" as Hitch put it. In other words, like Karen said, it isn't necessary for us to understand it. But without the MacGuffin, there would't be a story because it's the thing that gets the characters to take action.

Another MacGuffin in this movie is the mystery regarding what's transpired in the flower bed.