http://www.uesp.net/wiki/Skyrim:Lemkil

A farmer that blames his wife's death on his daughters.

He not only beats Sissel but encourages Britte to do the same.

Is it right to kill him considering that it's the only thing you can do to stop him from abusing his daughters?

Ooh. Another morality thread.

Unlike Grelod, it's hard to cause in-game accidents due to the lack of "insta-death."
I managed to develop a system where I gifted the kids large toys now and again until Grelod eventually tripped and died.

With this, I don't know how we could solve it any other way.

Where does it say he hits them?

Both the kids say that he hits them and it is pretty obvious with his dialogue that he treats them incredibly bad.

Bob, I'm thinking hunting accident if you can't stand murder. The man wanders outside, you mistake him for a doe, and the children are freed. Remember: your character doesn't know what you know.

Killing him is not enough. I also go the extra mile and reanimate him. I then kill him again. I then instruct Sissel on how to reanimate his cruel corpse. Goodness knows she might need to know some necromancy if her sister keeps up with the bullying. She's a quick learner. With my teaching, she'll be reanimating Draugr Scourge Lords by the time she's 16!