Why is it that Terry’s breaking point was being used to get into the Garden of Innocence (where Claudia wasn’t hurting anyone) rather than anything before that, like resurrecting her father (who had invaded Xadia and planned to take it over)? Claudia had done way worse stuff that Terry had no problem with.
Because Terry's loyalty was never really moral endorsement, it was personal. His whole arc is built on being the elf who left his own people and felt like he didn't belong anywhere, and Claudia was the person who saw him and accepted him without conditions. He offered her the same in return. So Viren's past crimes were something he could hold at arm's length: they belonged to a man he'd never met as a warlord, only as a dying, remorseful man trying to make amends. And "a daughter who won't let her father die" is a motive Terry could read as love rather than evil.
The manipulation broke that framework in a way the body count couldn't. Unconditional acceptance is a two-way promise, and being used (especially having his own trust and his own connections exploited to get to the Garden) meant Claudia hadn't been loving him back, she'd been steering him. Once that's true, the thing he'd been telling himself justified everything else collapses. It's also the point where he can no longer say the dark road hasn't changed *him*.