Protecting people from the harsh facts of life is certainly dangerous. People do need to learn to take the knocks life gives you and to understand the real value of kindness and decency etc.
Unfortunately, too often this reasoning is used as an excuse for treating others like crap; as if they are somehow doing them a service, when they're really just being an asshole. Life is hard enough as it is, no one should intentionally make it any more difficult for another just because they can't be bothered being pleasant. If you're being cruel, at least fess up to it; don't attempt to justify it as some sort of 'secret kindness' and pretend its something other than what it really is.
It depends. Sometimes people really are just being realistic, rational, or objective and that isn't "mean"...and in some cases people may even be snappy with someone because they're personally offended by how sheltered the adult they are speaking with is (notice I say adult, not child).
There's a difference between being a straight-up sadist, and just being a more realistic or even cynical person. Also, some people get annoyed by whiners when what is happening isn't really a big deal. In that case the perception of the whiner is that the annoyed person is being "mean" when in truth they're fed up with what they see as childish or weak behavior.
There's a big difference between abusing children, or being a person who intentionally delights in the pain of others, and what I'm speaking of. There are spiteful, wicked people who lack empathy or sympathy for their fellow humans...and then there are just people whose motives are different than what you may have considered.
For example, I've noticed that sometimes when people live under a great deal of stress, are hungry, or homeless they might be "mean." Mentally ill people can be "mean." Sometimes the more compassionate thing to do is to step back and ask yourself what's really going on with that person instead of labeling them as "mean."
Again, I'm in no way justifying sadism or the concept of children/teens bullying one another.