What are the repercussions of giving chems to Mama Murphy?

You just have to deal with the fact that she died.

You missed "if Preston or (reformed) Cait are close by they really dislike your actions" (not tested with others but I imagine most of them would) off your list of consequences in the instance you kill her.

There are upsides and downsides to being her supplier, if you supply her all the way up to the point where she asks for Psycho she won't die. Each time you complete a mission with her you'll gain experience. As long as you haven't killed her you have the option to get her to quit.

The repercussions of not giving her chems and instead convincing her to quit are that you don't get to use her wonderful psychic abilities, which do strangely include bits and pieces of information that allow you to avoid a couple of encounters that occur during the game.