NPR's Morning Edition this morning had the start of a series on undercover informants. The first account was of a guy who infiltrated a drug ring across the border in Mexico. He ended up being involved in the drug ring's 8-month reign of terror in which many murders were carried out.
Justice Douglas took the position that undercover informants were incompatible with a free and democratic society, and I tend to agree. The down side just seems too great.
The "ends justifies the means" idea is the same argument used in favor of torture. Some things like this just are so incompatible with our values that they should never be permitted.