When a celebrity or public figure slays it means they are really talented and loved for their personality and great looks. If someone slays the charts, they are so talented that they will usually be seen in the top 10 a lot.
To kill. Mainly archaic. Possibly an impressive feat of arms resulting in the death of the opponent.
Past tense is either "slew", or occasionally "slain".
"Slay him!"
He slew his opponent in a duel.
St George is famous for having slain a dragon.