But wait, if it's on Steam you just install the game and it launches.
Don't even try that.
Section 2, subsection G: Restrictions on Use of Software
G. Restrictions on Use of Software
[...] you may not, in whole or in part, copy, photocopy, reproduce, publish, distribute, translate, reverse engineer, derive source code from, modify, disassemble, decompile, create derivative works based on, or remove any proprietary notices or labels from the Software or any software accessed via Steam without the prior consent, in writing, of Valve.
[...]
Section 4. Online conduct, cheating and illegal behaviour
You agree that you will be personally responsible for the use of your Account and for all of the communication and activity on Steam that results from use of your Account.
Your online conduct and interaction with other Subscribers should be guided by common sense and basic etiquette.[...]
You agree that you will not create Cheats or assist third parties in any way to create Cheats. [...]
You acknowledge and agree that either Valve or any online multiplayer host may refuse to allow you to participate in certain online multiplayer games if you use Cheats in connection with Steam or the Software. [...] Further, you acknowledge and agree that an online multiplayer host may report your use of Cheats to Valve, and Valve may communicate your history of use of Cheats to other online multiplayer hosts. Valve may terminate your Account or a particular Subscription for any conduct or activity that Valve believes is illegal, constitutes a Cheat, or otherwise negatively affects the enjoyment of Steam by other Subscribers.
This is in the Steam Subscriber Agreement for when you purchase the game through steam. It forces you to check a box saying, quote, "I agree to the terms of the Steam Subscriber Agreement."