What's wrong with Philadelphia District Attorney Seth Williams? He won two elections as a reformer as well as convictions of corrupt politicians, pedophile priests, and assorted thugs. But he seems to have forgotten a simple rule of public service: Don't ask anyone for a favor. Williams' office in particular is too important and powerful to be compromised by special consideration.

Williams apparently broke the rule when he asked real estate investor and campaign contributor Robert Herdelin to give his ex-wife a deal on a home in Drexel Hill, as the Inquirer's Chris Brennan reported. She was paying $1,000 a month for a property that could fetch $2,500.

That casts a distinct shadow on the district attorney's image. For the public to trust him to enforce the law impartially, he has to set a high standard of personal integrity and independence, distancing himself from Philadelphia's favor-trading political culture. But Williams won't even answer questions about this suspicious arrangement. He owes the public at least that much.

The rental deal is the latest disappointment in what is proving to be a pattern of poor decisions by the city's top prosecutor.

Earlier this year, Williams declined to investigate a brawl involving John J. Dougherty, the politically powerful electricians' union boss. That sent a message that the connected could get a pass from the prosecutor. Sure, the feds were said to be investigating the fight, but it's the district attorney's job to investigate street violence no matter who is involved.

Federal authorities have also asked questions about Williams' campaign finances and his ex-wife's novel rent control arrangement.

Meanwhile, Williams' recently concluded investigation of the deadly 2013 Center City building collapse raised questions by targeting the least powerful people involved in a series of terrible decisions, putting two contractors in jail but leaving the property owner and others unscathed. Now the district attorney is fighting a court order to give plaintiffs' attorneys access to the cellphones of the architect on the job, the demolition contractor, and the city building inspector who committed suicide days after the collapse.

What makes all this especially troubling is that Williams is the district attorney who successfully revived corruption cases against Democratic legislators and a traffic judge after the investigation was ditched by Attorney General Kathleen Kane. And his investigation of the 2012 hose factory fire that killed two firefighters exposed systemic incompetence at the city's Department of Licenses and Inspections.

He also worked with former Mayor Michael Nutter, police, and the judiciary to reduce the murder rate and combat gun violence; reorganized his office geographically to target crimes more effectively; overhauled the charging unit to make more accusations stick; and increased use of grand juries to protect witnesses. The office's conviction rate has improved as a result.

That was the man Philadelphia voters elected twice. Now they may be wondering where he went.

Williams is due to stand for reelection again next year. He has little time to restore the public's faith in his judgment.