In life, you should never go around kicking innocent little kittens (or even mean and nasty kittens). You shouldn't throw a rock to break the window, or step on that person who fell down, or stomp on a plant just because you think it shouldn't be growing there. Never do those things in your daily life. But when you're writing your books, you should be doing them all the time. You're an author. And I hereby give you permission to give in to your cruel streak.
Being cruel to the characters in your books goes way beyond simply killing them. Killing a character is difficult, yes, but it can all be done with a few paragraphs, really. But making a character suffer? That takes a certain level of sadism, a particular brand of cruelty. It takes meanness. And you've got it in you.
The question is: can you bring it to the surface and put it down on the page for the entire world to see?
There are few true rules when it comes to writing. It's such a personal thing, and so many authors do it in their own way. But there are a few certain truths, and this is one of them: you've got to feel all of it. You should feel the happiness and the sadness. You should cry the tears and be laughing the laughs. If you're not doing this while you are writing, your writing isn't strong enough. This is absolutely true.
So when your characters are suffering, you are also suffering. It takes a special sort of sadism, then, to create that suffering. To, in fact, make it much worse. Until you reach the breaking point -- and maybe even go past it But that's what you have to do. It's always good to make characters suffer. They should face obstacles. They should not get everything they want. They should be disappointed, get their hearts broken, and they should ache. Because when you can create emotions out of absolutely nothing, you've done your job as a writer.
You've got to exercise your cruel streak, show off your dark side. Make people suffer. It's hard. But as long as you keep it all on the page, it's okay. You have my permission to be cruel...so run with it.