Forum:KB:Episode 7-8

Just a quick question im kinda confused on. During episode 7 kylo keeps talking to vader about helping him to the darkside. But wouldn't the first thing luke teach his students (including kylo) that even vader came back to the light side. 1, why isnt vader teling kylo that the dark side is not good, and 2, why does kylo ask for vader help if he knows that now he is on the light side?