IN some replay cams I see, they are shooting at me but their cursor is not even near my body and they are getting head shots. While I'm blasting them in their dome and they win the BR/DMR battle. I won't lie, I suspect them to be using something at times when I see the kill cams. Either that or they are host and I'm not.
What you are seeing is a lag issue, you see that all the time.
aim bots on xbox are a big pain in the ass to make, Just to use one on a Xbox 360, 1 you have to have a modded xbox, and 2 have a Aim Bot program made just for the game. every game has slightly different programing so they have to adjust it on a case by case biases.
Modded Xboxes are so widely banned now, that modding one to use online is like throwing money down the drain, they even have ways to tell if you have a modded controller now.
Aim bots are rare to see, there are a few tell tail signs that they are using them but I haven't seen someone use one since MW2, but a lot of what you think is a aim bot is just someone taking advantage of the games built in Aim assist, like in MW2 the quick scopes, worked because you just had to tap zoom-in and pull the trigger really fast in the general direction of someone with a Sniper rifle, the scopes Auto aim on to the other player when you first zoom-in basically lining up the perfect shot.
Halo 4 has a pretty heavy handed built in aim assist, much more so then the COD games, I think its because it's not a reflex shooter, so they can afford to do it without it drastically changing the game play. if they put the aim assist in COD that they do in Halo no one would ever have to aim.