Oh, no, I don't claim to know when the best moment to break your oath would be. I'm not a writer. It's just that, logically, If killing the goblins breaks your oath, killing the tieflings must break your oath also. If killing the tieflings doesn't break your oath, neither should killing the goblins.
All of these goblins are murderers who will murder again. The tieflings are innocent. Yet, are their lives worth less? I don't think Larian means to state this, so it needs to be changed 😵💫