あれが最後だったのかって英語でなんて言うの？

この場合、次のような言い方ができますよ。

ーI guess that was the last time I'll ever see him.
「あれが彼に会う最後だったんだな」

ーI took it for granted that I would see him again, but it looks like that was our last meeting ever.
「また彼に会えると当然のように思ってたけど、あれが彼に会う最後だったのか」

ご参考まで！