Abel felt relaxed after he killed Wizard Kilmer and Wizard Elliot. With the only two wizards that could use the "instantaneous movement" spell gone, there wasn't anyone he couldn't take on within the Assassin Union. Rather, even if others could use the "instantaneous movement," there was simply no way that they would help. Non-core members were not allowed to break into this area. That was just the way the security system here was built.
Head Commander Acton was still alive, yes, but there wasn't much that he could do with White Now and Johnson here. The teleportation circle was no more, and without someone to teleport him out of this place with spells, he was pretty much set on making his very last stand here.