I don't think that Julius is a bad guy, I am just assuming that Asta was shocked because he killed one of the two guys that were with Rades and Valtos. Though, I guess we can't really know until the translation comes out in a few days.
That's what annoying about them. Despite being useless they still ride in their high horses and didn't put into consideration the people that fought with their lives on the line while they let themselves get into a trap.
Where exactly did you get that one, if I may ask? At no point Asta remarks that there is something off with Julius.
There is certainly something fishy about Julius, but not enough to put him to the "dark side" in my opinion. He would have no reason to capture any of the White Night Eyes, unless he is associated with another villainous group that is. That he was not around during the incident is not really the issue here. It's common practice to leave the leader (or characters who could deal with the enemies easily) out during an attack, just to build up tension and to give the main cast and/or other characters screentime. The only fishy thing about Julius is that he waited for the White Night Eyes at that specific location. Which means that he either was aware of the attack from the beginning, didn't interfere for whatever reason and followed them with his uber-magic or he realized the attack later and tracked them with his uber-magic. For both cases, he could have his reasons without being the bad guy here.
WE know 4sure its someone from golden dawn--which makes me think its william or someone hiding in his ranks (kind of like an aizen situation)