I seem to recall an obscure (possibly optional) rule in 2nd edition, whereby you wouldn't improve your lockpicking ability by killing monsters. As I remember it, each of the four class categories (warrior, wizard, priest, and rogue) was supposed to earn XP in a different way. Warriors got XP for everything they killed, no matter how they did it, but the other classes only got XP from monsters if they killed them in class-appropriate ways. A thief, for instance, would get full XP from monsters they killed with a backstab, or defeated through some sort of trickery, and spellcasters got XP from monsters defeated using spells or other class abilities. Meanwhile, the non-warriors were also supposed to get XP for using their class abilities to defeat challenges outside of combat, so a thief would get XP for unlocking a door or disarming a trap.