Of real life experiences. Watching a show or reading a book is not the same as actually experiencing it. Your comparing apples to eggplants.
Ever felt any emotions at all when you read or watch something? Like, crying over a moving scene or feeling tensed in a scary, suspenseful scene. Those are real emotions elicited by a fictional story, a complete lie.
How's that possible?
As it turns out, your brain can't distinguish what's real and not real. That's why you get a rush of adrenaline when the character you connect with in a story is in danger, your heart pounding. They are lies, right? Why does that happen? They're made up people, so why do you care?
We're not toddlers or retards; we know what's real and not real, but our brains and bodies don't. That's why fiction is often used to instill some kind of lesson or truth since time out of mind. Kind of like a subliminal way of teaching you something.
Tragedy is often used as a cautionary tale for ages. But the question here isn't about the lesson, it's about whether watching something negative will instill you a negative energy. In other words, will it make you cynical and pessimistic if consumed a lot? Kind of like someone watching too much depressing news and becoming cynical and pessimistic. This is the only concern I have.
Even if that was true, why the fuck is 'becoming case hardened' a good thing? That's like saying I beat the shit out of my toddler for crying to make him a tougher man. That's callous. That's something to be avoided, not encouraged.
Becoming case-hardened in this context means to deal with stress and pain better. I was saying something about endorphin in the original comment. Like I said, when you get stressed or feel pain, your body releases endorphin to numb it down, so you're not in a complete agony. It's a survival mechanism.
For example: when you watch a tragedy and see how bad things are for the characters you care about, you get stressed, so endorphin is released to numb it down. Once it numbs all of it, you'll have established a new threshold for stress tolerance.
In other words, when you deal with a stressful situation in real life, you'll be more clam and composed and better deal with the problem at hand. It's basically saying, "I've dealt with harder shit than this, this shit is nothing. Now let's get to work."
Your toddler example is completely idiotic and out of context. Is your example about getting used to beating up toddlers real? Yes, I believe so. But that's also a matter of conscience and common sense, which is not what we're discussing here at all.
If you use that example and decide becoming case-hardened is a bad idea, then when applied to a completely different context like better dealing with a stressful situation, you would think freaking out like a bitch or getting depressed over it is a good idea.
There are many exciting pastimes aside from consuming a story that have the same process of raising your stress level in a safe environment and making you feel better at the end (extreme sports, rollercoaster, etc.). It's completely normal.