Lucas is the external factor. Ruu is a Familiar close to nature. When what looks like one of its manifestation appeared, he resumed what he's supposed to do initially, protect it. Since Shuya wanted to kill Lucas, Ruu disposed of him. Isa died because she attempted to stop Ruu from protecting him. (He read her thoughts and got rid of the hindrance first) Nessa is collateral damage.
Shuya is beyond saving. Nothing short of rewinding the timeline could save him. The Embrace don't work if someone's head is smashed into paste.
The Other Two, if Jack's OK with decending further into inhuman monster territory (As in into 'Dangerously close to snapping and killing everyone in sight every time someone pulls out a knife on him"), are probably savable... if they're willing to lose essentially all magical ability and become Kindred. Of course, this is implying that either Jack or Ryuu survive this
So why isn't he attacking Shirou?
Because currently Jack is in full on Berserker rage and the two are fucking each other up with supernatural power - but Jack's just grabbed Protean 4 and an extra point of blood potency - the repeated frenzies, drainings of vitae, and all that other fun shit accelerated his development of more blood potency.
Basically, Jack's claws at the moment, can sheer through shit that your average FMJ bullet has trouble with. While causing it to rot. And he's in Frenzy, so that's increased as well.
Daiki, really, at this point, one way or another, Ryuu will be either rendered disabled but kill Jack, or Jack will win and kill Ryuu but end up probably devouring whatever warm thing with a pulse he first sees because of all the blood he's spending in this battle.
