Killing guards, but not merchants...?

[Axlrose]

Throughout my travels (which is not very far), I've noticed that players slaughter town guards (especially in Rivervale) but leave the merchants alone. Is there any particular reason? I've noticed the merchants will not jump into the fighting fray - are they non aggressive even with aggressive behavior nearby? Or they do not offer neither experience nor loot?

Curious for curiosity sake.