You have an obligation as the DM to narrate exactly what the intention is unless you have a valid in game reason for doing otherwise that DOES NOT PLAY THE PC. Barring something like dominate or other mechanical effect, you have no right as the DM to tell me my PC does anything other than what I intend, or what he thinks, or what he feels.
If you want to tell me that my PC is afraid after missing a save to dragon fear, fine. If you want to tell me that my PC doesn't go into a dark room because you've decided to narrate that he's scared of the dark, that's not fine.
You didn't. You gave, "Deciding how to throw" which is hogwash. And you played the PC as "pausing his throw to look around at other stuff", which is also hogwash. You don't get to play the PC in any way. You only get to narrate what I decide modified by any in-fiction things.
For example, if I tell you that I am climbing the wall, you don't get to tell me that I am distracted by a bunch of kids crossing the street and don't climb the wall. You can narrate kids crossing the street while I'm heading to the wall and let ME decide if I'm distracted and want to stop. You don't get to stop me for that, though. You can tell me that the wall is greased and impossible to climb as I attempt to climb it, since that is not playing my PC or stopping my action of climbing the wall. It's simply auto failing the attempt due to a valid in-fiction reason, rather than giving me a roll or making it an auto success.
The system made no such determination. You did and you gave bupkis reasons for it. You auto failed his roll inappropriately(playing his PC) and then killed him. That's abuse of authority.
Were I at your table and you decided to roleplay my PC in order to stop an action that I had declared, I'd get up and walk out of the game. That's abuse of authority plain and simple.
Um. It's clear that you haven't been paying attention at all to what I have been saying. If there's a wall of force, than that would be, according to EVERYTHING that I've said here, be a valid in fiction reason for failure. That's completely different from you roleplaying my PC.