Is exposed wire dangerous if neutral is capped?
Just a minute. Be careful assuming those are hot wires. Non-contact voltage testers are notoriously over-sensitive and will read the whole box as hot if there's just one wire inducing voltage onto the others. I see that one of the black wires has a cap on it. He may have just disconnected several that he was working on that he didn't want to become hot. It's rare to have several different Hots all in one box. Usually, though not always, there's just one. 

I use one of two different  tricks with a tester like that to determine which wires are actually hot in a big cluster of wires. 

1, isolation method: You can dampen sympathetic signals by using your free hand to grasp the wire you want to test by its insulation a little further down from the bare end. A finger and thumb is all you need. Hold it away from the other wires and now lay your tester on it again. If it is hot it will still indicate, if it is not a hot wire it should appear dead.

2, Reduce sensitivity: lay your index finger on the opposite side of the tip of the tester when testing individual wires in a box. This usually will give 90% more accurate results and cut way down on false positives.

I know that box looks bad being uncovered and wires splayed out like that, but I still think your electrician left it safer than it appears. He probably thought he would be the next person in the attic, which was a wrong assumption, but I just don't think that he would have left hot wires wide open like that. If still in doubt, you can get a multimeter and test the bare ends of the wires for voltage against the equipment grounding conductors (all bare), or against the neutral.