Yeah, I think that's fair. It was deliberately shown as out of character. I'm struggling to believe those were enough buttons to actually cause someone to shoot someone but, then again, I was predicting a scenario like the Joker in The Dark Knight, what's his face in Skyfall, or Loki in Avengers where he deliberately engineers his capture and then controls things from the inside. I definitely thought they he could not be stopped unless he was killed (so, in that sense, Ward's call was the right one provided it was done spontaneously and not under orders).
I agree with your assessment that it was the right call because at the time it seemed like he had deliberately set up his own capture. So with that being said, was it really all that out of character for Ward to kill him? Ward kills people all the time when they are an immediate threat and there is no other options. He's a tactical guy, it's his job, it's what he's trained to do. Considering letting him go wasn't an option and the situation seemed to indicate that taking him into custody was just part of his plan, is this really that different?