Clementine can actually kill 2 people before Season 2; Lee and the Stranger.

In order for Clementine to kill the Stranger you must fail to strangle him as Lee, basically don't spam Q. He'll tackle you and Clementine will shoot him in the head.

Then in order for Clementine to kill Lee you must instruct her to do so instead of leaving Lee.

The only person you can kill is that guy in the green shirt who was harassing Christa. Basically do what the game makes you do, he'll die because of your actions.

When you meet Matthew you'll talk to him for a few minutes before Nick shows up and does his thing. You can actually grab Matthew's gun, assisting Nick in being a dumbass.

When talking to Walter about Nick he'll ask if he's a "good man" or "just like everyone else" if you tell him he's just like everyone else, he'll let him die instead of saving him.

When you have to choose between staying with Rebecca and Alvin and finding Kenny, go find Kenny. Once there he'll shoot Johnny before you guys move around the building. Tell Kenny to take the shot at Carver effectively getting Alvin killed.

When you get into Carver's office to turn on the speakers and play the "music" open his desk drawer and you'll find an old pistol with 1 shot. When you're shimmying behind him pop a cap in his face, assisted murder counts.

When you're walking through the herd chop off Sarita's arm so she screams like a little ♥♥♥♥♥ and dies.

When you find Sarah with Jane, leave her ass in the RV.

When you're crossing the ice lake and Luke falls through, give him covering fire. After Bonnie and Luke fall into the water don't break the ice, this will cause them both to die.

When Kenny and Jane are fighting let Kenny stab Jane in the chest, then shoot him in the face.

Stranger
Lee
Green-shirt-♥♥♥♥♥♥♥
Matthew
Nick
Alvin
Carver
Sarita
Sarah
Luke
Bonnie
Jane
Kenny

For a grand total of... 13 people.