Truth be told, he feels offended. By this cruise, by this man, by the boy in the red jacket and by the chicken. All of this is offensive on personal level, and he can't quite believe that they're making him watch this... disgusting display. As Kanesada halts with him, and mocks the clearly enraged chicken, Oikawa snorts; he's fairly positive it's impossible, but then again, chickens do have sharp beaks. It's just that they're usually rather docile -- or, at least, female chickens are. This one is a rooster (the feathers on his head kind of remind him of Kuroo's hair, really) and it's clearly out for blood.
Thankfully, not Oikawa's.
As it latches onto the boy's leg (Oikawa thinks he looks pretty familiar -- did he try to intrude to hang out with his daughter once? If he did, Oikawa is glad he didn't let him in), he screams in pain, falling straight onto his butt. The sound that the fall produces is... unnatural, at best. Instead of a sharp smack of bone hitting the wooden panels, a squelch can be heard, and then he sees... it.
"Oh my God, oh -- ugh, I'm going to throw up," one of his hands wanders to Kanesada's shoulder as he turns away, leaning on the man slightly. "Ohhhh my God, what did I do to see this, aaah! This is so disgusting!" As he freaks out, the boy continues screaming, as the chicken keeps to tear at skin and flesh. A sick smell of feces and blood rises in the air, and Oikawa has to pinch his own nose to keep it out. Honestly, he'd rather choke than open his mouth right now -- death seems like the better option.
He's nearly in tears by the time Flava comes down, commenting on letting the boy on the yacht. As stupid as the old man is, Oikawa has to agree -- who's even married to this guy? Whoever it is, he feels deeply sorry for their loss.
When he gets the courage to look again, he sees a massacre -- the chicken had left the boy's leg, but it looks positively destroyed, flesh and bone visible through long tears in his sweatpants. He's still screaming, the sound of his voice blocking out most of the noise, but something else catches Oikawa's attention -- namely, the chicken, who seems to be running after his daughter now. Ayano looks scared, trying to avoid suffering Shintaro's fate, and Oikawa feels panic -- he can't just let her leg get eaten, can he?
"Sorry, I'm going to borrow this real quick," he says to the stranger, grabbing the hilt of the sword fastened by his side. He pulls it out with one, smooth swing of his arm -- and while he can't be sure that the sword's even real, if he can hit the chicken with it, it should be fine, right? And just like that, he jumps into the crowd and away from Shintaro and the awful smell, chasing after the chicken. Ayano seems trapped, her only way out being jumping into the water -- he approaches the chicken from behind, sneaking up on it slowly, until it's within the reach of the sword to cut.
With a deep breath (he's not heartless -- killing an animal still makes him sad, regardless of how monstrous said animal is) he swings the sword once -- tears the chicken apart with one, smooth cut, improvising completely. He's seen sword-fighting in anime before, though, so really, how hard can it be? Aside from cutting the chicken, the tip of the sword gets stuck in the wooden boards underneath. He tugs on it a few times to pull it out clumsily, and when he does, people swarm around him, cheering.
"THIS HERO HAS SLAIN THE BEAST! GIVE IT UP FOR YA BOOOOI!" Flava yells, throwing his hands up in the air, and the crowd responds in a similar manner. He's surrounded, now, but his gaze wanders to seek the stranger -- he did take his sword, after all.
He still has to apologize and give it back.