Also, no one wants the memory of whatever a completely gone warlock is fixed in your mind for ever. You only do it because you want to be absolutely sure you are not killing someone who has been framed or something like that. I suspect the Merlin did it because it came with the job, I can't think many people willing to do the soulgaze. Also, if there was a chance that the kid survived, it was not a good idea to have him seen your very soul.
So, soulgazes are partial, but true. So if what the Merlin sees is too horrible, is enough. It does not matter if you also love music and cartoons.