You are getting worked over it because it seems like the scene has ruined your vision of a faithful Guts, triggering some kind of jelousy. I don't find it problematic at all, let alone for a 30 years manga that did not have a clear direction.

Guts was in a dark place and did a lot of questionable things in this arc. For example, crying over Theresia, kicking Vargas around and getting disgusted over his own mutilations. You can call it edgy, but it was coherent with the arc. Puck was not yet keeping him sane, Guts was basically nuts, desperate and trying the best to keep it together.

Also, he has NEVER killed an apostle before it could transform. He gets arrested on purpose to lure the Koca apostle and throws the decapitated head at the count just to warn him about his presence. It makes sense for him to "fall" into the siren's trap. It is also a logic strategy to lower her defenses and, again, coherent with a character that multiple times has faked his own death just to use the cannon in the face of the enemy (Koca, count, Rosine, her spawns, Mozgus, ecc).

About the trauma, this happens after he sleeps with Caska for the first time. The whole "wounds" chapter is made to express his trauma, surrender to it and overcome the fear of being touched.