You're right. The problem is DOJ needs to have a legal hook to go after Zimmerman under federal law, which would be either Zimmerman acting under color of law or interfering with Trayvon's civil rights (which would include, for example, using a federal food stamp card to buy skittles but would not include just walking down the street).
So their only angle is that Zimmerman was somehow a state actor acting under color of law.
Some of the Demonrats have been pushing a federal bill to regulate neighborhood watchmen.
Obviously they can't do this under the current view of the Commerce Clause, but they could do this as a civil rights law if they can show a pattern of local neighborhood watchmen violating the civil rights of people.
The Commerce Clause grants Congress the power to regulate local business activity if any part of it affects interstate commerce, if the aggregate of activity of that industry has a substantial effect on interstate commerce.
Congress acted within its power to protect interstate commerce in extending coverage of Title II of the Civil Rights Act to restaurants serving food moving in interstate commerce. Congress had had ample basis to conclude that discrimination based on race by such restaurants burdens interstate trade.
In Heart of Atlanta Motel v. United States, the Supreme Court found that the Act was a valid exercise of Congress’s power to regulate interstate commerce in prohibiting hotels from discriminating against customer based on race.
http://www.lawnix.com/cases/katzenbach-mcclung.html
They don’t need no stinking “acting under color of law”. “Civil rights violations” will suffice for any purpose or person.