This game (duke 3d atomic edition) has mouse aiming so I do not want any aim assist.
I am willing to write commands like it was needed in blood.
Can anyone help me?

Didn't find a setting for that in the regular game... so... you're going to need some work to do. First, you should get your Atomic Edition working under eDuke32, which provides many more features and bugfixes, including one to disable aim assistance.

http://www.eduke32.com/

Download the program from there. It'll be in a 7zip file, you may need to use WinRAR or 7Zip (which I recommend) for it. After that, find the folder where you installed the game. If you have the icon that the GOG installer made for it, right click it and click "Open file location" then go back one folder, and extract the contents of the 7zip you downloaded into the game folder, then launch eduke32.exe. After that, a launcher window will appear, feed it a resolution and what not and start the game.

Enter Options, then Game Setup, and change AUTO AIM from REGULAR to NONE. Congratulations. You have disabled auto-aiming and gave yourself a better version of the game. From now on, you should launch the game through eduke32.exe to enjoy the added features. Hope I helped.