A point not being made here. Follow the timeline: - Lydia is captured. Carl meets her. Sexual hijinks ensue. - Alpha picks Lydia up. Carl throws hissy fit. Carl runs after her. - Alpha picks Carl when resting. He travels with the whisperers to their camp. - Alpha explains to Carl how, by showing how they lived, she expected he'd go back to the communities and carry the message that the whisperers need to be left alone. Carl refuses, due to the fact that Lydia is being raped by the whisperers, with her mother's consent. - Alpha claims she must learn more about the communities, and leaves to the fair. Next we see her is fully dressed in front of Earl "learning" about the fair. - Unbeknownst to Carl, Lydia and Alpha, Rick has found out that Carl has left and travels to get him back. He gets stopped in the middle of the road by a group of whisperers. - Rick arrives to the whisperer camp. Meets Carl, has a discussion with him. Alpha arrives, bloody machete in hand. - Alpha takes Rick to see the horde. They go back to the whisperer camp, have a fight, Alpha tells rick to take Lydia and leave, and "not to cross the border of their lands. He'll see it when he gets there" - They get to the middle of the road and see a fight has broken. The result seems to be the heads-on-pikes of a number of community members.

From this timeline we can see that Alpha decided to kill all of them without even knowing they would come for Carl. Nevermind Rick going to get his son, Alpha just murdered 12 people with no justification whatsoever, given that it would have happened even if they did not come to get Carl.

So on the who started it question, it's a toss-up between Carl and his hormone rage, and Alpha being a nutjob. While she had told the communities to stay out of her land, and Carl directly disobeyed her, she also insisted at a point that Carl should take her message and tell the communities to leave them alone, which he flat-out refused due to not wanting Lydia to stay with them. That was the trigger for her to go and "learn" about the communities in the first place.