[Bungleau]

First off, I agree with Wyv -- killing is good for those who deserve it, not for innocents.

That being said, the critical thing is to slay him in one blow. If you do that, he doesn't have the chance to cry out to tell the others what you've done.

Not sure if the other toad in the area will have an impact. You might want to be out of his line of sight before you do the slaying.

You might also take the groupie into a room all by himself to slay him, out of sight of the others.

And of course, save beforehand just in case this doesn't work...

As for the oath, sadly the game doesn't really take it into account. I wish it did... really. You'll be able to wield the Mavin at the end whether your heart glitters with gold or reeks evilly with darkness.