There is an inherent lag I'm ever game as a result of the players connection speed, hosts, and communication between host and server. Imagine eight people, each a thousand miles apart, sending data to the server, server sends it back with a packet update, your console renders the feed, and you react. It's only hundreds of seconds between console, host, and server communications, but its enough to cause a perceived differential between how how the server tells the host, tells your console how to render the animation. I'll bet you a hundred dollars that on the other players console, he was was looking right at you when he fired. Its not cheating, its lag. In fact, the slowest component of the game is the player. We react in tenths of a second, while the console is tens of times quicker. Look up gaming lag on wiki, they can explain far better than I.
But if you want to see actual cheaters, play team slayer on weekends early in the morning against two or more forerunners. You'll see grenade sticks that do nothing, one shot dmr kills, and players taking three and four pummels before they die.