Attorneys defending Prince George's County police attempted today to portray a key witness against them as a hard-drinking, drug-using ruffian who had a predisposition to commit the armed robbery he claims police illegally lured him into committing in 1967.

David E. Wedler, 34, one of six plaintiffs in a major $9 million civil rights trial in federal court against members of a so-called police "death squad," acknowledged under questioning by Deputy County Attorney Michael O. Connaughton that as a teen-ager in the mid-1960s, he burglarized a building, owned at least two handguns, punched a policeman in the nose, used heroin and marijuana and was ejected from a Langley Park shopping center drug store for rowdiness.

Evidence of a "predisposition" by Wedler and others to engage in crime has become a key element in the police defense against accusations that they set up a series of five phony robberies and burglaries in 1967 by directing informants to recruit participants for the crimes. In the five incidents, police waiting at the targeted stores shot and killed two suspects, wounded a third and arrested seven others.

Police have denied in the complex, 11-week-long trial here that they instructed informants to recruit participants. Further, they contend that the criminal "predisposition" of the participants--as reflected in their arrest and conviction records and their association with other known criminals--bars them from claiming police entrapment.

Barnet D. Skolnik, one of four attorneys representing the plaintiffs in the case, has countered that "predisposition" is largely irrelevant since the police action of instructing informants to recruit participants is illegal, regardless of whether the recruit is a criminal or a "choir boy."

Wedler, a tall, rangy man, testified today that he had partied with a loose gang of friends and was "high" on liquor and codeine-laced cough syrup the night of Nov. 26, 1967, when he went with two companions, William C. Harris and Sydney (Chico) Hartman, to a 7-Eleven store in Chillum and attempted to rob it at gunpoint.

Police were staked out at the store, however, and when the three started to flee, Harris was shot and killed and Wedler was arrested. Hartman, it turned out, was an informant who had tipped police to the robbery plan.

Wedler was ultimately imprisoned for 28 months in connection with the robbery. He, like the other plaintiffs in the case, contend the "death squad" stakeouts violated 14th Amendment rights of due process.

In addition to Wedler's teen-age run-ins with the law, attorneys for the police officers had attempted earlier to introduce evidence to the six-member jury that Wedler was convicted of manslaughter in 1971, four years after the alleged "death squad" incidents, but Judge Herbert F. Murray allowed the plaintiffs to exclude it.

In that case, according to court records, Wedler was originally charged with murder in the Hyattsville shooting of Donald Caputo, 24, a brother-in-law of William Harris in the current "death squad" trial here. Wedler said at his 1972 trial that the shooting was accidental when the two men tussled over a rifle. The jury convicted him of manslaughter.