At that point, the off-centered Alpha fully accepted Negan as a Whisperer, though she definitely didn’t anticipate that his reaction would be to murder her where they stood.
In comparison, he calls Alpha’s own emotional indifference as being falsified, since she still clearly longs for her daughter’s presence.
Murder her he did, though, all to get himself in better graces with Rick’s group.
This inspires one of Negan’s most iconic comic book deliveries, in which he speaks to his wife’s death causing him to become emotionally stagnant, which allowed him to do everything he did as a Savior without harsh feelings of remorse.