I have seen loads of cheaters in online gaming.
Back in 1995-1996 when I was playing duke nukem 3d online on wireplay, there were bright skins, a bright map hack & there was even someone who made a colour aimbot to use with the bright skins. (on a DOS game!)
so cheating in online games isn't really new
I think up to 1996-1997, there was more of a respectfull online gaming community tho. As soon as news of 'pro-gamers' started and Sujoy, fatal1ty etc were getting press coverage there was a sudden surge of cheaters.
I think around the same time, counterstrike was released and I'm pretty sure that is and was the worst game for cheaters.
For some reason kids seem to think if they cheat online they will get noticed and be able to become pro-gamers. Not sure how they think that tho, unless they think they are going to be able to take the cheats to LANs.
Quake3 was quite bad for cheaters aswell tho, esp when OSP became popular

PB helped combat it a bit, but it never really stopped the people who could program the cheats.
I think kids cheat for a few reasons.
Some do it to annoy people, recently on cs:s this has been seen a lot with the 3day passes. Kids coming onto servers using their sound scripts to play annoying music over the comms and just blatently going round aimbotting.
Some do it to feel like they are good at something, having others looking up to them.
Some do it to become pro-gamers (not sure who they're trying to kid)
Some do it because they can or because they think other good players 'must' be cheating. I find these people really bad and have encountered loads who do this, they have done it to me.
With punkbuster back in the quake3 days, we caught so many people and on a quake3 server I was an admin on I wrote come cvar checking scripts and caught 3 admins from a gaming network. 1 of them was the head admin, who later dissapeared from quake3 (strange that?)
I don't like the way people call cheaters hackers either as most of them are probably 13yr old kids who just download cheats to use and don't have a clue how to program or anything.