Its hard to say for sure.
I've experienced the child behind who kicked the seat while the parents did nothing, until I reached around and squeezed the little brat's leg hard enough they decided not to kick any more.
I've experienced the person who decided to apply toenail polish that stank out the cabin until the air staff asked them to stop.
The trouble with a lot of "unacceptable'" behaviors is more down to the airline than anything else. People complain about "manspreading" but when your knees physically won't fit in the space ahead there are only so many options available. You don't have to be particularly badly overweight before you start to spill into the adjacent seats.
It's all very well saying you should pay more for a larger seat but when business class can cost many times what economy class costs it's one of those situations where it's easy to say what should be done when you don't have to do it. And even if you do pay extra for a second seat (thereby almost doubling the cost of your trip) you potentially then have to pay more to make sure your second seat is actually the one next to you.