Here we go again. Washington's alleged master burglar and accused murderer of Dr. Michael Halberstam, who supposedly caught him in the act, has asked, through his lawyer, for a hearing to determine his mental competency.

The man charged with the murder of John Lennon was in a Bellevue mental ward almost before the gun smoke had blown away.

The clear implication in that Bernard Welch and Mark David Chapman will plead innocent by reason of insantiy when they come to trial for murder. With luck, one or both may even manage to avoid trial altogether.

The very possibility is one of the things that drive me batty about our criminal justice system. I'm willing to suppose that Welch and Chapman -- if they in fact did what they are accused of doing -- are crazy. Welch's lawyer suggests that "there might be some pattern of compulsive behavior which might have its roots in mental illness."

Perhaps. The multimillion-dollar stash police found in his elegant home clearly suggests that the alleged burglary spree wasn't the product of economic desperation.

Chapman, who allegedly bought a gun, quit his job in Hawaii and flew to New York for the express pupose of killing a man against whom he had no rational beef, may be, as a New York cop called him, a "nutcake."

But if a non-lawyer may legitimately entertain such questions: so what? Shouldn't it be the first order of business to answer the fundamental question: did they do it? And how can we do that unless they are brought to trial?

I'm jumping the gun, of course. There is no reason to suppose -- yet -- that they won't be tried.After all, the lawyer for David Berkowitz, accused in the "Son of Sam" killings, found a couple of psychiatrists willing to pronounce the suspect mentally incompetent and incapable of participating in his own defense. Berkowitz was tried and convicted anyway. Chapman already has been transferred from Bellevue to a jail cell.

But still, our unwillingness to separate the question of whether an accused committed the act he is charged with from the more problematical question of why he did it strikes me as wrongheaded. So does the notion that an otherwise criminal act is not a crime if it is the product of mental disorder.

You need look no further than last week's newspapers to see the mischief that can follow from this notion. I refer to the story, out of Chicago, that the man accused of beheading his girlfriend and mailing her toes to Gerald Ford and Alexei Kosygin has been recommended for release from the mental institution to which he was committed after the court found him innocent of murder by reason of insanity.

Psychiatrists say he is sane now, and, therefore, there is no further reason to keep him confined.

I'll grant that the acts of which he was accused were the product of a sick mind, if only because they were so singularly bizarre. But it wouldn't take much to convince me that nearly all of the most outrageous crimes are the products of sick minds. But again: so what? I'd be willing to let these crazies serve their statutory time in a mental lockup instead of the state pen, but that's about as far as I'd go.

My faith in psychiatry to cure these sickies is about on a par with my faith in the ability of prisons to rehabilitate the officially sane.

Sometimes the inmates of prisons and mental institutions get better; sometimes they don't. I count it a worthy thing to try to increase the number of inmates who do improve.

But why should that have any bearing on whether rapists and murderers and madmen serve out the time the law prescribes for their offenses? After all, we imprison people for what they have done, not for what they are, or what they fail to become.