"That would do it." It does fill in the gaps. Lucifer was probably drowning on rage at that point, something Istredd is well aware he feeds from. He wouldn't be able to tell friend from foe at the time. It's not an excuse and Kyle has every reason to be upset and distrustful, no one should be concerned a friend would turn on them. But it makes logical sense.

Istredd frowns at him. "Do not let people stab you, Lucifer." That bothers him more than the idea of someone kissing him. Well, about the same level of bothering. "I don't care what you've done or that you'll be fine. No stabbing of my partner allowed." Lucifer already knows that Istredd is very protective of him. He would have probably attacked Kyle if that did happen and made a scene. So it's for everyone's benefit it didn't happen.

"I'm glad to hear you apologized without prompting though." That would be unheard of before he did it with Wilhelm. Another sign he was changing. "Kyle's smart. He probably understands you wouldn't ordinarily act that way toward him."