Why did Lara and Sam stop being friends?

Why did Lara and Sam stop being friends? After Mexico, where she killed a man for the first time, Sam’s mood swings worsen, and her relationship with Lara deteriorates until the two friends are barely talking. Sam hasn’t told anyone about her visions of Himiko.

After Mexico, where she killed a man for the first time, Sam’s mood swings worsen, and her relationship with Lara deteriorates until the two friends are barely talking. Sam hasn’t told anyone about her visions of Himiko.