Because you would have killed me in their defense, or is this politely oblique death threat meant to reference something else?

[There's no need to specify who 'they' are. This entire conversation comes back to Frisk, one way or another. But even though she's completely certain Chara knows their real name, she did promise them not to use it around other people.]

I've done far too much damage by simply assuming I knew the answer, before, so it seems best to ask.

[She doesn't seem at all frightened or angry or upset with them. Serious, yes, she's not ignoring this and she knows that it's important, but she's responding calmly, tone even and sincere.]