It's like a high-wire act. One slip and you fall. And there's no net."

The speaker was an intense, sad-faced man using the pseudonym Sandy Stillwell as he described the fragile shadow life of a police informer to a federal court jury hearing testimony here today in the Prince George's County police "death squad" trial.

An admitted house burglar and one-time street hustler, Stillwell spoke of being whipsawed by police who constantly pressured him for information about planned crimes and threatened to throw the book at him if he failed. He said his one great fear was--and continues to be--that his role as an informer might be disclosed to the criminal community.

"There's only one way you deal with an informant," he said, " . . . and that's eliminate him. It's like a cancer. You got to eliminate it before it kills you."

In his three days on the witness stand here recently, Stillwell, 34, left a bewildering trail of assertions, denials and contradictions, freely admitting he lied to authorities from time to time to protect his informer status.

He is the last of five informers and ex-informers--all of them convicted petty thieves and some of them with a history of treatment in mental institutions--to testify in the "death squad" trial, now four months long. Much of the complex case rests on their credibility with the jury.

The first four testified against the police, contending that in 1967 Prince George's County detectives instructed them to solicit criminal acquaintances to participate in a series of convenience store robberies and burglaries where police would be waiting for them.

Two men were shot and killed, another wounded and seven others were arrested by police in incidents in which police have been accused of instructing informers to solicit participants. Survivors of the two slain men, plus two of the arrested men, filed a $9 million civil rights suit against three county police officials, including current Assistant Chief Joseph D. Vasco Jr., contending that the officials masterminded the so-called "death squad" incidents 15 years ago. Police have denied the allegations.

Stillwell, unlike the other informers, who were brought into court unwillingly from prison or self-imposed anonymity, testified today that he wanted to appear on behalf of the police to counter claims of one of the other informers, Ronald LaVelle.

He disputed LaVelle's testimony that Vasco and then-Det. James Fitzpatrick asked both Stillwell and LaVelle to get suspect Richard Charles Schoenian "involved" in a burglary of Greer's liquor store in Bladensburg on Sept. 3, 1967. On the contrary, Stillwell said, Schoenian's participation was at his own initiative and police never asked him to recruit Schoenian.

But under cross-examination by attorneys for the plaintiffs, Stillwell acknowledged that he told Maryland State Police just the opposite when they were investigating the "death squad" allegations in 1979.

At one point, he said he lied to state police because he feared they would divulge his role as an informer and he wanted to protect himself from reprisals in the criminal community by making it look as though he had been coerced by county police into recruiting participants for crimes.

At another point, he said he lied to state police because he had recently been beaten by a county police officer after a drunk driving arrest and that "added to my enthusiasm" in lying about the "death squad" allegations.

Nevertheless, Stillwell said he wanted to testify now in support of Vasco and Fitzpatrick, who had "always treated me honorably" and to deny that they ever asked him to recruit participants for crimes.

When he finished his testimony today and was excused from court by presiding Judge Herbert F. Murray, the slender Stillwell said as he walked out, "Now I'm going back into the woodwork."