All right so I know most of the opponents arrayed against your kill team are hordes, even some of the tougher ones. Larger enemies such as carnifex are obviously individuals. What I am interested in is how have people portrayed chaos marines in their games? I can see them being thrown at the PCs as a tough horde maybe, alot of novels show a few loyalist marines slaughtering dozens of tainted astartes with almost no effort.

I think if they are included they would be better off played as enemies that can go face to face with the PCs and make the players really make an effort to beat them. There is no shortage of hyper deadly opponents for players to face, so would it matter to design legionaries this way?

Personally I would want my players to sit back after beating them (or dying in the attempt) and feel like it was a great game. So if any of you have ideas, experience, or anything add please do.

I think combat with CSM should be special. There is 10,000 years of hatred between them, either because the CSM are fighting the long war or because the SM has been inprinted with his original legion's legacy. CSM will be taunting the deluded fools and trying to get them to change allegiance whilst trying to kill them. It should be personal and feel personal!

Have the CSM introduce themselves, name SM's they killed or battles they fought (maybe even the Eternity Gate or Istvaan if they are ancient) and get 'in their face'. If SM's are epic warriors, then so are CSM as their dark counterparts. And ideally, some CSM will survive the encounter and escape to return often to annoy the SM.

None of this is possible if they are portrayed as a horde. The only way I see that happening is if the CSM uses human heretics as ablative armour (as done by the Astral Claws).

I was considering creating them using the PC rules or attempting to find a copy of Dark Crusade and building a chaos marine character. I have never played DC so i'm not sure how well that would work.

A BC CSM made at equal xp level is a boss level encounter at lower ranks. Since their base is only 8k, they get an additional 5k xp just to match a starting DW. Since BC doesn't have a rank system they can have the highest level talents giving them a huge advantage. Throw in their fate points (infamy) and you're looking at maybe a round and a half to drop each KT member depending on weapons.

If you want a return tormentor, you could use cultists or legionaires as ablative wounds and set a wound threshold that would be the GM's trigger to withdraw back into the warp. After seeing the engagement(s) and determining if the KT could actually defeat the CSM Tormentor, then an epic "Boss Fight" can be crafted and presented to the KT....with an appropriate amount of Reward and Renown to be granted after the encounter. If the KT would be exterminated, then I wold suggest it just be a recurring story element antagonising the KT. Perhaps at some point the epic encounter could/would be eventually faced and maybe at the expense of a PC or two but NOT the whole KT....that would put me off as a player that had devoted so much time and energy into this game.

I see no problems with elevating the difficulty level but to keep the players playing the game....I would NOT kill them off. Dice rolls can be fickle and things may not always go according to plan. As a GM, having a contingency or adapting to the game conditions is just as important a part of the game as it is for the players. The game was designed and developed for the PC to have a career and progress and advance. It also isn't ALL about fighting and destroying things.....that is why there are Skills and Talents that aren't directly related to Combat in the game.

YMMV

The novels are pulp-action for the most part that don't make any logical sense (you should not slaughter genetically engeineered combat veterans that are hundreds of years old with daemonic gifts). That's because a novel does not have to be internally coherent, whereas an RPG does. It is a bad idea to use them as an idea of what the RPG should be like, since they are completely different genres. IMO.

I can see them being thrown at the PCs as a tough horde maybe, alot of novels show a few loyalist marines slaughtering dozens of tainted astartes with almost no effort.

A lot show the reverse. Many black library novels - as with star wars novels and indeed most sci-fi/fantasy novels - suffer from what can best be described as 'protagonist syndrome'.

Using the PC rules is perfectly appropriate if you don't want to get Black Crusade - many chaos marines are recent renegades and many of them are just as dangerous as any legionary veteran.