I know that Klaus swore revenge but he never took it out, also Elijah was very sympathetic about Elena's brother dying but what about his own? Yes I know Kol was kind of bad but if oyu think about it he was only trying to stop them from uncovering Silas to save the world, unlike Elena and the others who were looking for the cure for their own selfish reasons. Kol was trying to be the good guy, if he hadn't been killed and they listened to him then, Silas would not be back and Bonnie would not be dead.