It's hard. It's so hard to kill off characters you actually grown to like, for JK Rowling it was probably difficult to kill off Tonks and Lupin. Or even Sirus I cried reading those deaths and of course one of the Twins. It was like Aww my Ginger-haired duos turned singular.
Sometimes it needs to be done though. You have to give an MC motivation, or a reason to seek revenge. In order to do this something bad must happen to them. For Bambi his mom had to die. For my characters, well, her roommate/ best friend dies, and I'm going to Kill off a few more, but it will make her stronger. More willing to say F-off world I'm doing things my way.
But when Killing them, (Characters) do it with a purpose not because you hate being inside their head anymore or they do something to make you personally mad. Do it with a purpose.