PHILADELPHIA (LNS)- A former FBI informant now doing drait resistance work says he changed his mind becuase of what he heard at rallies the FBI assigned him to cover. "My downfall as an effective informer carne when I found mself agreeing with what I heard said at the various rallies," Robert F. Tatman said at a rally sponsored April 16 by the Vietnam Veterans Against the War. Tatman, 24, said he reported on about 10 events from 1964 to 1966 and was paid about $150 for his efforts. He showed reporters copies of an FBI document stolen from the FBI office at Media, Pa. Tatman's name appeared on the document as one of a number of sons suggested as possible informants for a conference af war résisters at Haverford College in August, 1969.