... Um, credit Lea with the lie, she is the one who put the cloak on her and enforced the markings on her then shot a bullet at her and it didn't hurt her.
Susan was supernatural-savvy enough -- by this time -- to know about faeries & iron. She'd have known the (lead) bullet would be fully-resisted by the faerie magic, but iron/steel weaponry would not be.
An excellent demonstration, though, of just how powerful Lea's magic was, if
not facing iron!
... Harry repeated that, though he had serious doubts about how effective Fae magic would be against steel.
No, Harry specifically claimed
Martin's (steel) knife wouldn't hurt Susan, in that moment. He wasn't alleging any general/overall safety, just the one instance... and presenting as a "fact" something he knew was NOT a fact: faerie magic cannot blithely ignore iron. Not even Queen Mab's magic at Arctis Tor, the heart of Winter's power; overwhelmingly more power than Winterfae magic at the heart of the Red Court's power, in front of the Red King and the LoON's!
... Point is, the bit about whether or not the knife would hurt her was totally irrelevant, Susan wasn't worried about her safety at that point.
No.
"Susan would have attacked anyhow, so Harry's off the hook for any/all lies trying to get her to attack" is your logic, here.
No.
Harry was still trying to manipulate her into the attack.
He lied to her, and the objective of that lie was to get her to attack.
I agree that he didn't need to -- the only reason he even had TIME for this lie was that Susan was processing a moment of shock and realizing the truth, things were finally falling into place about Martin as she saw the depths of his betrayal. Once her gut -- that had trusted Martin a few moments before -- finally "got the memo" about Martin's real role, her attack was a foregone conclusion.
That does not excuse Harry the lie, the intention to manipulate Susan into attacking. Missing the kill-shot when you attempt to murder someone is still assault with a deadly weapon; attempted rape is still sexual assault; etc.
Just as Susan "only needed to tell the truth" to manipulate Harry to
Do Anything to rescue Maggie... Harry "only needed to tell the truth" to manipulate Susan to
kill Martin and
fully-Turn, thus providing an
alternate bloodline for the Curse to destroy.
Susan intentionally set Harry up, put him at extraordinarily-dire risk.
Harry intentionally killed her.