Quicksilver Science!

[Lord Torath]

There's no real scientific answer here. The real world doesn't work that way. In Tome Raider, Laura turns a knife around, and that reverses its direction of motion so it strikes the thrower instead of the intended target, which I assume is kind of what you're talking about here. It really comes down to who's writing the story, and how they want things to play out.

If someone is physically grabbing things and moving them, the acceleration they apply to the object should add to the velocity vector (after converting the acceleration to a velocity), resulting in a "diagonal" final velocity vector. If you want to assume Quicksilver is redirecting bullets in a vacuum, he's going to need to overcome the momentum in the original direction, and impart a similar momentum in his desired trajectory for the object. There's nothing keeping the bullet moving in its direction of travel other than its inertia. Merely rotating the bullet around the vertical axis isn't going to rotate its velocity. It's like how the Star Furies in Babylon 5 could turn to face perpendicular to their direction of motion to shoot targets to the side or even behind them. Unless you're talking about an active rocket, which accelerates itself. Even then, turning it perpendicular to its original trajectory will not result in a velocity vector perpendicular to its original one. It would move off at an angle between the original direction and the final orientation of the nose.

If Quicksilver is trying to do this in atmosphere, The What If linked above is relevant not just to the bullet, but to the air molecules he's moving through to get into position to move the bullet in the first place. That scene in Over the Hedge where Hammy is casually strolling along faster than the DePelter Turbo IR lasers while weaving between the leaves? Yeah, no. He needs to weave between the air molecules, too. That doesn't stop me from really loving that show, though! One of my favorites!

If you could magically teleport and reorient the object, you could conceivably reorient its velocity and inertia as well.

It's like the ending of the How it Should Have Ended: Antman:
"...How did you carry a 20-ton tank in your pocket?"
"Oh, you know how it goes. Mass.... Molecules..."
"Convenience!"
"Convenience!"