Kaito ended up growling at Rantaro. He was growing angry with him, although he didn't know why. In reality, he wasn't angry at all. The only reason he was arguing with the other to begin with was him finally breaking down from keeping all of his emotions bottled up for so long. The stress manifested itself as anger, causing him to continue arguing.
"I know that Shuichi died for a bit! He's my best friend, you know! I was with you! Did you forget that…!?" he yelled before slapping Rantaro. He was just that angry. "Now shut the fuck up!"