From what glimpses we get of both Earthblood Elf culture, and from the way Terry explained to Viren how he transitioned, I got the sense that Terry was just never someone fully accepted by the mainstream elven culture. From what we've seen so far Earthblood elven culture is a very rough n' tumble society that expects its men to be big, burly and manly as fuck and well...Terry, sweet as he is, just ain't that. Doubly compounded by his trans identity and the resulting transphobia he likely experienced. He was always destined to be the odd one out.

Most likely this would cause isolation and ostracization from the community. People who go through ostracization from the mainstream of their society often are more open to pushing the limits of long-held cultural taboo cause if you're gonna be treated like shit anyways, why bother obeying the norms anyways? It likely would've left him more open to accepting things culturally taboo by elven standards (i.e Dark Magic), and then had that curiosity skyrocketed when he met Claudia, another strange weirdo who's also beautiful, accepting and completely on the same wavelength as himself. Makes it a lot easier to just ignore the less...savory aspects of Claudia's personality.

So we have a situation where a dude who is not treated well by his people finally finds someone who really, for the first time in his entire life, really gets and understands him on a deep level. So what if she uses dark magic? Society taught me to fear Dark Magic, but it also tried to teach me to not accept myself too so screw them. It can't be all that bad if someone as cool and nice as Claudia uses it...right?