I am not sure what Jimmy stood to gain by killing Karen, it would give Henry more incentive to flip. You would want both of them in the same place at the same time to whack them both and decrease the chance that they may escape.

I felt that if anything, Jimmy was gonna hold Karen hostage.

One thing that a viewer should look out for is possible symbolism. After all, this is a movie and it's directed by a legendary director.

Toward the end of then scene, you see a sign that reads "One Way" and another that says "DON'T WALK".

A one/way walk to death? I don't think Scorsese just happened to have that in here, he's a very detail-oriented director.