Actually he was the only who actually tried to do something after. He just enjoyed it as well. And it has less to do with morals, and more to do with messing with someone he cares about. Like it or not, he cares for Karen.
You are actually going with "I hit her because she is mouthy" defense.
Karen is far from a monster. Is she manipulative? Yes. Is she self-centered? Completely. But there is some clear reasoning for how she is, and she most certainly isn't a killer.
Mandy is manipulative, self-centered and a lunatic. Horrible things have happened to her, but that doesn't suddenly entitle her to kill someone because she knows Lip doesn't care for her like he does Karen. Before Karen even showed up this season Mandy decided she was Lip's wife and was going to run his life.
Karen manipulates people to get what she wants. Mandy kills people to get what she wants.
Let's see Karen has:
1) Slept with a drunk Frank to piss her dad off.
2) Took a crap on her dad's grave...mind you, I thought the guy was a jerk and do not think this is a deal braker for the character, but she certainly does not come off as likable in that moment.
3) Married Jodie just because she was bored and wanted to take a dig at Lip, which is confirmed when she grows bored with Jodie and throws him out.
4) Forces her mother to choose between herself and a special needs grandchild who Karen would leave to the system, even though Sheila wants to take care of him. She then runs away when she doesn't get her way like the spoiled brat she is.
5) Only returns home when she lyingly and manipulatively has Hymie taken away from Sheila.
6) Manipulates Lip (who also has very little respect from me) into banging her in the bathroom while still dating Mandy.
7) Tells Mandy about it when Lip chooses Mandy, because she is a spiteful, evil woman.
8) Says, quite honestly, she plans to make Lip impregnate her with a sabotaged condom, so that he takes her with him if he goes to college.
9) Encourages Lip not to go to college.
All around, Karen is the most vile and unlikeable character in the series. That is saying something when Frank called Child Services on his own kids this season.
She does not love Lip, she only loves how she can control him and use him for her own entertainment, like she did with Jodie before she got bored and like she did with Sheila and is now doing again after she took Hymie away from her mother. Karen is a horrible, horrible human being. And in a show where your protagonists steal, intimidate and use violence when they can (did you hate Fiona for paying Mickey to beat the **** out of Cousin Patrick or for Lip planning to beat/maybe kill a child predator until he learned it was a hot woman teacher?), it makes Mandy running her over seem almost justified.
Granted, in real life something like that is horrible and Mandy was in the wrong. But how is Mandy a horrible girl when she ENCOURAGES Lip to go to college, even if he doesn't take her?! She wants Lip to make something of his genius and to do something with his life. Karen is a cancer who is only intent on sucking the life out of Lip, like Monica and Frank did to each other. If he stays in the South Side, which he will, that is his future on the show.