Dishonored - K&M or Gamepad?

Keyboard and mouse is superior in my opinion. You can use numbers as shortcuts for spells, and mouse is always great for aiming.
Advice, if you have a thumb button in your mouse, use it for spells, and leave the right click for blocking. This worked very well for me.

I always go controller besides competitive FPS multiplayer.

It really depends on your preference. If you're fairly skilled with a keyboard and mouse as well as a controller, you should try out both ways and see which one feels smoother to you. I personally started from console gaming and crawled my way up to PC gaming, so I had little experience gaming with a keyboard and mouse. After a small bit of practice, I quickly got used to it. I still prefer certain games with a controller, though.

I would say the only reason to use a gamepad is if you want/need the precision of movement speed/direction, or if you happen to be more comfortable using it than mouse and keyboard. Other than that it will only make actually turning around and pointing at things precisely, slower than you could do with a mouse. That said I've not played Dishonored, so I don't know how important these factors would be in the game. To be honest, an ideal set-up would be using the mouse for looking around and the gamepad for the actual movement, but I'm not sure if this hybrid technique is even possible, or remotely feasible to do in a comfortable fashion if it is possible.

It's a first person game, so of course, keyboard and mouse.