Regarding Youkkai *ahem*, the evidence supporting him being a merciless indiscriminate killer is - well, I don't think he DOES. I mean, in his and Goijyo's Burial arc (ERNT ERNT SPOILER ALERT READ NO FURTHER IF YOU DON'T WANT TO BE SPOILT OUT THE WAZOO!), Gojyo's lying on the floor bondaged while Youkkai's beating the crap out of the yakuza. He's a sitting duck. An easy target..

Hakkai doesn't touch him.

Ditto with Goku in Kami's Magical Bondage Gourd. Goku is right there, and an instinct-driven killer would have chased him down and tried to mangle him. The outcome of THAT fight um...that's anyone's guess :D;

I think Hakkai is afraid - nay, he's TERRIFIED he might hurt his friends by mistake. He perceives himself as a very dangerous person, and he works exceptionally hard to keep himself in check.

Dammit, Hakkai, get a hug or something!