Hey!! Why Katherine kill John? PLS :)

Well, John was turning out to be a huge threat to the Vampire race. And that damn ring didn't make it any easier. But I think was it really neccasary for Katherine to have to cut his fingers off? That flash was...unpleasant. But then again, Katherine is most likely sadistic. I hate her. But I guess she saw John as a threat to her. Plus, Killing is what most Vampires do.

Maybe because she knows he killed Pearl. She was supposed to be her best friend, so maybe she wants to revenge her. But I'm not sure if Katherine actually care about Pearl

i have 3 theories
1. because he killed her bestfriend pearl!
2. because he killed all them vampires that came out from the tomb, obviously some were her friends, and also because hes killing vampires left right and centre, hes endangering them.
3. this one is my opinion ;p, because isobel ordered john to kill stefan and damon right? well vampires can listen in on conversations, maybe ones only close to them im not sure, but i think that she wanted to stop john which is why she killed him, so theres quite a few possiblities ;p

This one is easy. John was a loose end.

Katherine's whole reason for killing the Tomb vampires was that she didn't want anyone to know she was alive. After they were all dead, the only people aware she was alive were Stefan, Damon, Elena, Isobel and John. She had her reasons for keeping Elena and the Salvatore brothers alive, and John and Isobel were the only ones who knew it was Katherine that orchestrated the attack on the tomb vampires which made them more of a priority. She was fond of Isobel as we saw, and Isobel was useful to her. John was the only one who had outlived his usefulness to her, and was more of a danger than an asset, so she decided to get rid of him. Simple at that