The ncr just told me to kill the brothhood, and i dont think tht should happen. Any ideas
oh and will they help me if i do save them

just talk to the brotherhood elder instead, they'll give you another option so you don't have to kill them.

There is a way to have the NCR and the BoS form an alliance, and if you do this the Brotherhood will help out during the battle at Hoover Dam. However, you must leave Elder McNamara in charge while you're doing the Still in the Dark quest. If you help Hardin oust McNamara, Hardin refuses to even consider allying with the NCR, and you'll have to destroy the Brotherhood if you're going to side with the NCR.

The only other way to save the BoS is to side with Yes Man and tell him you want to ignore them. The other two factions (Mr. House and the Legion) want them destroyed under any circumstances.