["But I don't blame you. It wasn't your own fault."

That's what Archer wants to say, but he doesn't want to let himself soften too much. Especially not to her. Still, Archer at least wants to confirm to himself this Illya was indeed better off somehow. Even if that meant there really was nothing he could have done for her.]

I'll be happy to see you there, then. And don't worry about that, alright? If I held any ill will towards you, I wouldn't have responded to this.

...try have a good birthday, alright? I couldn't live with myself if I ruined a child's birthday party somehow.

[That is nowhere near the worst thing you have done in your life, Archer, and you know it.]