Kaiba actually takes several steps back, not because he wants to show hesitation--but because this is...it's more than he's ever come to expect from Yuugi. The tone he takes, the way he demands to know whether Kaiba has ever experienced anything like he has?

It raises ire in the taller teen, but it's still kept at bay while Yuugi berates him. He's still processing that. Yuugi's...killed? Yuugi has seen people he cared about dying? But when Kaiba got here...there was no Yuugi, nor anyone who he knew from home so...

...how long has Yuugi been here?

It probably would have gotten through to him, except for Yuugi's pivot. The gambit that he actually thought might have gotten Kaiba on his side was to suggest--

What the hell are you talking about?! You want me to eat parts of you?!

I'm not so deranged as to try and eat the flesh of a man!

Though he is deranged enough to eat a poisoned cupcake apparently.