This kind of thing is unfortunately, all too common in every country (ok some countries moreso than others, due to cultural differences... for instance, definitely more common in China than say Singapore, even after accounting for population / country size differences), countless cases occurring every day / month / year. It's just that with the advent of camera phones / internet / social media, this phenomenon is becoming more well known.
I said it before and I'll say it again : #1 It's (mostly) not the mistress' fault, she's (mostly) an innocent victim, the primary perpetrator is the husband who is at fault for not being faithful, the secondary perpetrator in this 'crime' are the wife and her lynch mob gang. #2 Lynch mobs are (almost) never justified, karmically and spiritually speaking. The victim being lynched could have been framed, set up, and maligned. Even if she's truly a mistress, but she doesn't deserved to be lynched, no one does. Being lynched traumatizes and scars the mind and soul, the victim ending up psychologically and spiritually damaged and (falsely) feeling unloved by the world and universe. The lynch mob incurs severe karma for participating in the lynching (of course, actual severity varies widely across cases, fortunately these 'China mistress getting whacked by wife's gang cases', are *relatively* mild, compared to severe lynches involving torture, loss of limb, being burnt alive (you can easily find videos on the internet of mob lynched victims being burnt alive).