My older cat killed my kitten!!?

I am in complete shock that this happened. I have a 1yr old cat and had a 4mo old kitten. They were brothers, but from different litters....I am shocked bc they have been together for 2 1/2 months and all of a sudden this happens. My older cat is the nicest most laid back cat ever. They have always gotten along...playing, sleeping together, eating together....they were always okay with each other. They sometimes would play and it would get slightly rough and then it was over and they were friendly again. So i was taking a nap one day and whn I woke up I went upstairs in my bedroom to find my baby cat laying on the floor dead!! He had bite marks and wounds on his head and neck area and was bleeding pretty badly. My older cat was also in the room sniffing around and acting like nothing happened. What the heck just happened??? How can they be cool for so long and now this??? I dnt understand and I am just heartbroken over it....and to make it worse my older cat looks around for the kitten still and I guess doesnt even realized what happened. I need some answers