One of the great things about working with other people's characters is that there's a well-loved person already set and ready to go:  just make up interesting things for them to do.  One of the bad things about working with other people's characters is that you have to leave them a lot like you found them.  They can't get married or die or have any other major life-changing event happen to them.  But if they're not undergoing major life-changing events, then why is your game interesting?

One way to make it interesting is to add some characters of your own, people who CAN die.  Another way is to change something internal about your borrowed characters, give them experiences that will test them or deepen their characters in some way.

Still, if players know that marriage, death, and other large changes are off the table, it makes things feel less consequential.  In "Medical Problems 2," Vesemir, Eskel and Lambert are sick, and the illness is grave enough that they could die.  If the player makes the wrong set of choices, they should die.  But Vesemir, Eskel, and Lambert don't belong to me, so I can't kill them.  My "bad" ending has them sent to a sanitarium, where they will be gravely ill for an indefinite period of time -- gravely ill until CDPR puts them in another game, in other words. :-)  

Funny.  I love these guys, and it makes me sad that I can't kill them. :-)