Alexander was worried… Billy just began to see the girls as members of the opposite sex instead of kids, and then something like that happened. Alexander had his suspicion that Billy did something with magic to help Kate. Still, it was the first time that Kate declared that, so he could understand her side of things, he also would try to help others if ln the face of death, if he were saved by someone, but being stubborn like that without a clear plan wasn't a good idea and that annoyed Billy very much.