Okay, I think I can make an argument in favor of Gustang.
I think he may not have tried to go for the kill until this chapter, and that's the reason his abilities have been so underwhelming (because they weren't meant to be lethal). He was saying that Traumerei was no longer his friend. Throughout the chapter, it seemed as if he was still trying to convince himself to kill him.
And in the previous chapter, Traumerei said Gustang was toying around with him (before absorbing everything back).
Maybe it's a bit of coping to justify why he's been so underwhelming during this fight, but I think he can do decently more than what he's shown so far. The mindset he had until this chapter was kinda holding him back, maybe.
(Also for the sake of my agenda, I need to find ways to shorten the implied gap between him, Traumerei, and Blossom this chapter).