I knew the first one to be mentioned would be the kerb stomp.
Not sure if its the most disturbing but recently I saw a European (think it was Dutch?) film called Schemer about a group of teenagers who isolate a girl in their friendship group and turn the others against her, and they decide to kill her, the disturbing thing is how long and messy the death scene is, it takes about 6 minutes, one of the boys attacks her in the car and she doesn't realise what he's doing at first then she starts fighting him off and screaming and shouting for help, the others are all looking at each other nervously not sure what to do because they thought it would be over quickly, they stop the car and she is still fighting back and screaming and starts smashing on the horn beeping to attract attention, in the end one of them gets a rope out of the boot and three of them tie it round her neck and strangle her. There is no blood or guts and no music playing in the background but it comes over quite horrific because you feel like that is what would actually happen if a gang decided to kill the girl, she wouldn't go quietly she'd fight for her life, and it conveys the uncertainty and confusion of the attackers. It is also disturbing because these are supposedly her friends, so you see how she can't understand what is going on and starts to realise they are actually trying to kill her.