Wonder what Eb meant by this

[vincentric]

Maybe the Staff itself is sentient? Harry's charge set off a self defense protocol beyond Eb's control? Eb was holding back - but the Staff doesn't. Maybe the black veins on his rams are a threat to take over the wielder as well. Need great power and will to hold it back - when you project power and will it makes advances. Then you need to make it retreat. Elric's sword Stormbringer?

He wasn't using the Blackstaff when he fought Harry, just his everyday staff.

[morriswalters]

Eb had his gun spell loaded and ready to fire. It's very similar to Rudolf and his loaded gun. I suspect this is intentional on Jim' part.  The juxtaposition draws two very different emotional responses because you expect the one and are surprised by the other.

“Oh, lad,” he said, tears in his eyes. “Oh, lad. Didn’t think you were going to come at me again. Didn’t think it would trigger.” I could feel his hands on my face, distantly. “That’s why you were so big on teaching me control,” I slurred dully. “You’re barely holding it together yourself.” “I’m a hotheaded fool,” he said. “I’m trying to help you.” “You knew you were losing it,” I said weakly. “And you kept going anyway. You could have backed me up.” Blood came out of the hole in my chest in rhythmic little spurts. “And instead it ends like this.” Shame touched his eyes. And he looked away from mine.

Butcher, Jim. Peace Talks (Dresden Files) (p. 307). Penguin Publishing Group. Kindle Edition.

I felt Rudolph. Felt his terror. His agony. His confusion. His humiliation. His remorse. His sick self-hatred. I felt them all as if they were my own. I saw myself through Rudolph’s eyes, huge and vicious and deadly, implacable as an avalanche.

Butcher, Jim. Battle Ground (Dresden Files) (p. 212). Penguin Publishing Group. Kindle Edition.

[Mira]

Not totally surprised though by either.  Eb was showing signs of being on the verge of being out of control emotionally from the beginning.  He was upset when he arrived at Harry's apartment, that went completely over Harry's head or he just blew it off.  Both struck emotional nerves and it went down hill from there. 

Rudolph was way predictable, on a number of levels..