the ncr are harrising jacobstown but i dont want to help jacobstown i want to wipe them out is thier a quest for wipeing them out?

No quest that I know of but that shouldn't stop you from getting in some target practice.

Why would you want to kill Marcus?

There's no quest, just kill them if you really want to (though I agree with above poster, Marcus is really awesome, why would you want to kill him )

I played an abomination cleanser and he killed everyone in Jacobstown...the mercs didn't do anything, just stood there.

Marcus will eventually tell you that there is a gang of mercenaries harassing them and to deal with them. During this quest I'm s ire you can help them kill Jacobstown or do it yourself.

whats so good about marcus does he help ncr i only want to help ncr

Marcus is just a cool character with a deep story rooted in almost all the Fallout games. I also like what the Muties are trying to do by creating their own town. When Mr. New Vegas was talking about it I envisioned something like Black Mountain, but I was happy to see an actual town.

He helped the chossen one in fallout 2 save the world and destroyed the enclave oil rig

I don't know what everyone is talking about, there is a quest, its called "Do It Yourself". You find it right when you attack a super mutant in Jacobstown and another one sees you.

You want to kill Marcus