spoil ?

I don't think so, Ootsuki killed people directly and Yuuki caused a lot of deaths, but they were still able to enter oneness.

or maybe it's because he attacked an other person who has that power(〜￣△￣)〜 ?

I dunno, my guess is that either he's just too freaked out and can't concentrate, or Makoto somehow stopped him. ┑(￣Д ￣)┍

I don't remember, did Yuuki or Ootsuki kill people with their own hands (this was a stabbing with blood and all) or only from a distance?

Its been a little while since I read the whole manga, so I couldn't swear to it, but I'm pretty sure it was all done with Oneness.

I think it may be the fact that Makoto is the original, he brought Yuuki into the 'oneness' who inturn brought them, so if Makoto dies their connection to the oneness ceases to exist? Just a guess