The initial attack on Lex is never fully explained. Teague wants Lex dead so he can't control Clark. So he sends an assassin to... scar Lex's chest, so that Clark will see the symbols, so that Clark will go to Montreal, to encounter Teague. It would have been simpler to pay someone to paint the symbol on a wall or in a crop circle and kill Lex, and such a plan has the added advantage of rendering dead the one person who is close to gaining the secret. in fact, Teague does try to kill Lex later in the church, so it's clear he wants him dead.