Scott acting like Scott is verily subjective. During NXM Scott never really felt in charge to me. It felt more like the Charles, Jean and Emma show for a good chunk of it. I did feel Scott was much more in charge in Whedon's run and frankly Emma really never felt like a co-leader. More like a trusted adviser deeper in his council than the others. The calls were ultimately his. That's already a change from his NXM behavior. That's influence.
Emma and Jean had a disproportionate effect on Scott. Show me any other character that had that much on a impact on a main character in the X-Men as they had on him. You won't find it.
All Decimation did was radicalize the methods that the characters were already leaning towards. Logan leaned into homicidal rampages. Guess who else did? Yup, Scott. This clearly shows a break from his previous behavior. And please don't suggest that Decimation was the most peril mutants had ever been in. The O5 and the Brotherhood had been throwing themselves at each other in battle for ages prior to this and they represented most of the mutants anyone knew to exist in the Silver Age. The entire population of Earth has been threatened countless times (including all the mutants) and yet Scott had never once advocated for proactively wiping out their enemies. Something had changed and sorry, it wasn't the stakes.
But what caused the radical change? It can't be the level of crisis because that's been seen countless times before. There was more going on here.
Talk about hedging your position. It's fine. I don't really disagree.
I'm saying she'd already had an influence on Scott that goes back to Morrison. I'm not talking about responsibility. I'm not trying to blame Emma explicitly for what Scott did. I'm talking about the factors that mould perspective and decision making. Emma was one of those factors. Jean would have been a factor too, but she'd have been pushing Scott subtlety in a different direction. I don't see how any of this is controversial.