That's all we know for now.
After reading the first 5 volumes I am guessing that Hayato, at least in the past, tried to approach Yukino but failed. Hachiman probably piss him off because he sees Yukino opening up to some guy with a rather twisted attitude and solves problems through questionable means.
I agree that Hachiman suits well in taking on the role of an anti-hero.
Allen got me hyped for v6, lets see if I can finish it today
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