QUINCY, Ill. (AP) _ A 30-year-old doctor was convicted Friday of lacing co-workers' snacks with arsenic by a judge who said he was a danger to society and told him, ''you have clearly performed evil deeds.''

Circuit Judge Dennis Cashman told Dr. Michael Swango, ''It is indeed a sad day when you have the mind and the intellect to do great deeds.''

The judge found Swango guilty in a two-week bench trial of six counts of aggravated battery, for which he faces up to 10 years in prison on each count. Cashman set sentencing for June 12.

The charges stemmed from the poisonings last fall of doughnuts, tea and soda Swango served co-workers while he was employed as a paramedic with the Adams County ambulance unit. He was acquitted of a seventh charge.

Swango worked as a paramedic while he was awaiting medical licensing by Illinois and Ohio after a year-long surgery internship at Ohio State University.

Prosecutors had contended that Swango had a fascination with death that led him to administer the poison.

Among examples cited as evidence of that fascination was his cheering the man responsible for the 1984 massacre at a McDonald's restaurant in California in which 21 people died.Swango said such comments represented ''gallows humor'' used to ease the tension of emergency medicine.

Cashman said he did not understand Swango's motivation.

''I look at this evidence and I ask myself why,'' he said. ''The people don't have to prove why and I don't have to find why - but it would sure help if I knew why.''

Authorities in Ohio say Swango is under investigation in Columbus in the deaths of about a dozen OSU Hospital patients during his internship from July 1983 to June 30, 1984.

''This verdict may cause some people in the Columbus area, who have believed this investigation has been a witchhunt, to come forward if they know anything,'' said Edward Morgan, an assistant prosecutor from Ohio's Franklin County, who attended Swango's trial.

''These verdicts here today indicate to me and my office that we're certainly on the right track,'' Morgan said. He said OSU's campus security force, which was heading the investigation of Swango, had not completed its probe.

Illinois has suspended Swango's medical license. Ohio has begun proceedings to take the same action.

But Thomas Prunte, a spokesman for the Ohio State Medical Board, said Swango's conviction won't automatically mean revocation of his physician's license in that state.

''It would just be a basis for further action,'' he said.

None of the six paramedics was injured seriously by the tainted foods, laced with arsenic-based ant killer.

Cashman revoked the $5,000 bond Swango posted after his Oct. 26 arrest, and ordered him held in the county jail pending sentencing.

''I don't feel I can jeopardize the people of this community or any other,'' the judge said. ''I have every reason to believe you would do it to anybody else'' if given the chance.

In a 45-minute courtroom explanation of his verdict, Cashman said he was convinced Swango lied during his seven hours on the stand Wednesday.

Swango had denied being anywhere near paramedic quarters at a Quincy hospital on the day a cup of arsenic-laden tea was found there.

In final arguments Thursday, Assistant District Attorney Chet Vahle said ''an incredible array of circumstantial evidence'' pointed to Swango's guilt.

Swango's attorney, Daniel Cook, contended that the state's evidence was a string of easily explained circumstances.

But Cashman said a key piece of ''damning evidence'' was a nearly full bottle of ant poison seized by police at Swango's apartment that did not bear a label. A doctor trained in the use of toxic substances would not leave an unlabeled container of poison lying around, he said.

''It's obvious to me the label on this bottle did not just fall off,'' said Cashman, contending Swango ''had gotten it ready for his next unsuspecting victim.''

Swango said he had 13 insecticide bottles - some empty, some partly full - because his apartment was infested with the ants. He said his dwellings while a medical student in Springfield, Ill., and an intern in Ohio also had ants, and that he saved the bottles for use with a home chemistry set.

After the verdict, Swango and Cook were ushered out of the courtroom through a back door, followed by Swango's mother and a woman friend. They declined comment.