If all sin is equal in the eyes of God should I kill the store clerk of the shop I rob?

While He does not consider one sin to be worse, He is not saying "Go rob a store and shoot the clerk!" He's stating, for example, that lusting after a woman is just as bad as having with her outside of marital bounds. His standards are very high. And there is additional punishment. Either way you look at it, you're going to be punished for both sins. Not one.