Good thing Shulk is a touch more levelheaded, he was able to switch off his " Kill every last one of them" Mode.

In Three Houses, Dimitri, the prince who has been racked with survivor's guilt and PTSD after a massacre called the Tragedy of Duscur where his parents and friends had been killed, had just found out that fellow student and princess of the neighboring empire Edelgard, was "responsible" for said tragedy. And he takes it about as well as you'd expect. Of course, the truth is more complicated then that but the prince needed Byleth to reel him back to sanity to understand that, otherwise he would end up becoming a monster of a man.

The idea is that Shulk went through similar events at the attack at Colony 9 with the loss of Fiora and many of it's residents, where he would swore to destroy all Mechon. Fiora becoming Mechon and the existence of the Machina was of course a game changer for him, but if he hadn't hatred for the machines go, he could have easily went down the same path of destruction like Dimitri.

Think Shulk if he became more like Egil.