Secret Service

On the levels Rogue Agent and Habeas Corpus use ONLY the Taser on every Secret service agent you encounter. They will be wearing either black or blue suits so will be easy to spot. The other enemy on the Habeus Corpus level can be killed using the normal weapons. These people will be wearing green combat trousers and black body armour so are easily spotted when pitted against both types of people on the level.

However be warned there is also another achievement in which you have to use only the taser and no other weapon on the habeus corpus level so It is up to you whether you use the taser on both types of enemy or wether you use a mixture of taser and normal bullets.

All the Secret Service agents you kill on the level Expatriate are now classed as being an enemy so no longer count against you so you are free to kill them.

This achievement will unlock once you complete the game.

You cannot kill a single secret service agent (the guys in black suits and white shirts) throughout the game, the only exception is on the last mission, Expatriate, you are free to open fire and kill them. Just carefully watch your shots, on Habeas Corpus strictly limit yourself to using the taser or stun gun, this way you don't kill anyone by accident. This will also help you get the achievement White House Electrician as well.
If you kill one, quick quit to console and reload from checkpoint