Why did Matyas have a phobia towards blood/colour red? (spoilers)

We learn that his phobia stemmed from being in the bath with his brother Thomas who one day cut off part(s) of his genitals. But in that flashback we see Matyas carry on splashing in the water as if nothing has happened. He does not seem peturbed. So why would he later have such a phobic reaction?

my vessel is magnificent and large and huge-ish