If a wizard shape-shifts themself in order to kill someone without leaving forensic evidence....
That's starting to get close to the line. I would concur that it's not a First Law violation, but Wardens would start taking a more active interest in the character's activities.
Now, if the wizard shape-shifted themselves for some other reason (espionage), got attacked, and killed the attacker; definitely not a violation. Technically, it's no different than a boxer or martial artist using their Fists to kill someone; it may well be illegal, but not to the White Council.