Title.

I beheaded Idolaf Battle-Born and put his head on a plate in front of his father Olfrid as he sat down for dinner

Soul trapping and killing Paarthurnax are the cruelest things you can do. I'll never do the latter.

Killing Paarthurnax. Granted, I didn't know anything about TES and it was on my first character. I just wanted to see what the Blades offered me.

Never again.

I can't be cruel. I've tried and I just can't.
Although if people consider soul trapping animals/creatures/monsters to be cruel then I do that. I kind of have a philosophy on that one, an interpretation of how I think soul trapping works. I'll explain it later when I'm not tired but part of it is that I pretend that it doesn't steal the full soul or it steals the energy used by the soul to be in the physical realm, steals that as it passes on to wherever it is going.
I can't remember if there's any in-game books which specifically lay out what happens to a soul when it is trapped.

I also soul trap REALLY "bad" people if I come across a black soul gem.

Killing Paarthurnax. The Blades are [censored]. If I didn't need their help, their heads would be on stakes outside of Skyhaven.

animal's energy is only used. Black souls energy is used too but the consciousness goes to

DG spoiler

In which case I don't consider using the animal energy to be cruel.
Animal spirits go to Hircine, don't they?

I don't know about animals, but Werewolves do.

No just werewolf spirits. I don't think animals have a spirit like humans do. I think their soul energy just dissipates. Maybe returns to Aetherius, and just disappears if used for enchantments.

I can't find anything solid in lore online so I think this is a personal opinion choice, much like in reality.

I don't think the energy would just disappear. Energy doesn't die, etc. I guess it would remain in the enchanted item until it was used up. I think it would then wind up back in the natural cycle somehow. This is all me making up stuff on the spot though, just opinion.

Well, I wouldn't apply real life physics to this, but no the lore isn't very clear on it. I think it is more than likely though that it just gets recycled via the dreamsleeve.

But the dreamsleeve is sort of pointing to the idea that the energy does not die but is merely changed so really, it's not just me applying the idea of real life to it.
edit: though I see the dreamsleeve itself is up for debate on the interwebs.

Killed that girl at her wedding for the Dark Brotherhood quest to kill the emperor. I felt alful about that but it was the only wany to be able to go after the old assassin armor, but dand I felt bad about that.

I know, its why I said it would be recycled there. I'm just saying in general it isn't a good idea. Like when someone asked why sunlight reflected from the moon doesn't hurt vampires.

Sun physics was brought up for some reason, but the sun in the game is just a hole to aetherius, not a 'star' like we think.

Rage killed Paratus, than reanimated him, and killed him again. It's cruel but he totally deserved it.

It's still not a bad idea. A bad idea would be to stick to it rigidly and not accept magical explanations put forward by the writers and developers.
In terms of the sun not being a star, people will just have to accept that is why real life explanations would not work seeing as it is not the same. However, sunlight is warm which will melt ice and make the plants grow. That function of it still is consistent so it isn't necessarily weird to look for a little sense if there is no magical lore explanation.

Kind of off topic now though.

Stole Braith's sweetroll.

In all seriousness killing Parth, even on my evil character I felt like such a horrible person for doing it that I reloaded a previous save.

Doing Dark Brotherhood quest line.

Defying Alduin, Chief of divines, praise be.

Even more cruel, is killing his children. Tahrodiis Joore!

Aye, but the cruelest? Helping wicked Valdar with his scheme. *spits* Veistul nikriin!

Kynareth if any.

Hircine does not lord over animals. He just kills them. And turns sapient mortals into monsters.

That's a fair point. I just got through re-doing Ill Met by Moonlight so I have Hircine on the brain.

I get more and more creative with my cruelty every time I play, so it's hard to keep track. I detest the people of Skyrim more than any other game so far, probably due to the fact I found them to be very apathetic and artificial in their own special way.

My most recent one involves the two smiths in Windhelm and a certain someone's reanimated corpse.

http://i.imgur.com/hR5aLUv.png

Hheheheeeheheheheheheheehehehheheeeheheheheehheheheeeehee

*5,000 year old spiteful Dragon Priest giggling like a creepy undead school girl*

Removed Serana's clothes and hit her with the Mace Of Molag Bal every time she gets in my way.