Yeah, it doesn't get in the way of my enjoyment of the episode, or Alpha's final scene with Negan. But I simply would've added something else in. If he tied her up, have him tell her he's working undercover to sabotage the Whisperers the same way Dante was at Alexandria. It wouldn't have taken long. He could've even left her a knife to cut herself loose. I just had problems with it all. At the very least we needed a little more and it wouldn't have taken more than a couple of minutes of screen time.

 

I wonder if:
He did it to avoid the chance of Lydia witnessing him killing alpha.

To keep her safe from the mayhem and chaos in a place he could return to and tell her alpha is dead but not that he killed her.

To protect her from any roaming whisperers who might be outside.