Generally, for the crew as for anyone else, protecting oneself comes first; helping others is second.
There is only so much a cabin crew can do. Before takeoff and for landing, they will obviously secure the cabin, passing through the aisle one last time and checking everything to be in order before taking a seat themselves and buckling up. If a foolish passenger finds it wise to unbuckle the seat belt after this, how will they ever find out? And even if they knew, if they were to get up and deservedly admonish the pax, they would in essence sacrifice themselves and all the rules-abiding passengers for that one unruly: the cabin crew is no longer secured and rather prone to injuries, more than likely leaving their exit uncrewed in case of need.
If someone gets up on a quest for the toilet or starts to raid the overhead compartments, he will most likely be shouted at from the cabin crew stations; the maximum cabin crew can do is call the flight deck and report the cabin no longer secured - actions may vary thereafter and both options are not really desirable. A go around with its rather high acceleration is not always preferrable to an expected smooth landing.
What happens if someone gets injured due to his own stupidity and despite clear orders from the crew may depend on the company and also area of jurisdiction. Here, the company will firmly stand behind its crews and cover for them in case of trouble. Also, courts will most likely adopt a "common sense" standpoint and lay the blame squarely at the offending passengers feet if it can be made plausible that all the regulations have been complied with from the side of the crew. In other areas of jurisdiction, it may well be that procedures are examined and the company or even crew may be blamed for neglect.