If I can just throw in my two cents before the ethics discussion ship takes off, my speculation for what may have been motivating
@Thetan, taking her at her word, is something that has motivated many, many people who have interacted with Chris over the years. Many people, myself included, find Chris's antics and the trolling amusing and fascinating but have a desire to see Chris improve himself. However, "wanting to see him improve" and even "feeling sorry for him" is not even close to being on the same level as "wanting to be his friend." Wanting to be someone's friend typically requires a big emotional investment in another person, like someone we've known personally for years or a stranger whom we greatly respect: Chris doesn't fit either of those criteria.
It's like charity. Someone asks you if you want to help starving orphans by donating a dollar a day. "Sure," you say. But then the person asks, "If you want to help the orphans so badly, why don't you go to the playground with them every week and listen to them talk about their emotional problems?" Most people would say, one way or another, that they're just not interested in making that kind of commitment.
People have been trying to help Chris by white knighting for as long as he's been Internets famous. You can see plenty of examples on the CWCki, or hell, just look at the comments on his Facebook statuses over the past two years. People who like Chris and have even met him in person will give him good advice, and he just blows it off. When he's lonely, you'll tell him that you're here for him, and he'll say "Yeah, but what have you done for me LATELY?"
@champthom summed it up well:
This isn't me being an A-Log but from my experience with Chris, being Chris's friend is asking for trouble. I'm not saying people shouldn't be friends with Chris but friendship with Chris is, as someone else in this thread said, a lot of baggage. If you're Chris's friend and you're a woman, the expectation is that you're either going to enter into a relationship with Chris or you're going to set him up with somebody because "that's what friends do." Or you're expected to give him pity sex because he's EVER SO LONESOME. You have to worry about being doxed if you slight him or if he does something stupid (like he decides to tell the Internet "I TOTALLY HAVE A FRIEND, HER NAME IS x AND HER ADDRESS IS....") and you'll be perceived as a white knight to would be trolls which draws their ire. Friendship is supposed to be a mutual arrangement where both parties derive some sort of benefit from each other, but with Chris, it's mostly a one sided relationship. You're only a friend as so much as you can be a grade A ass patter and enabler.
In short: Being Chris's friend sucks.
I'm pretty sure Thetan has, in the past, posted publicly on Chris's Facebook with genuinely good advice, and Chris ignored it. Lots of people have tried doing the same and met with the same reaction.
I think Thetan, as someone who's followed Chris for a long time realized that 1) Chris generally ignores good advice from people he doesn't know, and anyone who attempts to give it to him is wasting their time, and 2) Chris is easily fooled by people pretending to be other people. Thus, she came up with the idea to pose as people Chris would trust in order to get around that obstacle. It's like entering a cheat code (presumably for Christian Weston Chandler's Adult Chronicles). However, while Thetan ended up planting some good ideas in Chris's head, she realized that doing so required methods that many people would find, frankly, bizarre and dubious.
Just an analysis I've thrown together based on what everyone has said so far, but it seems like a reasonable interpretation of Thetan's initial motivation; how not wanting to be his friend does not preclude having genuinely good motivations; and why she eventually felt guilty about it.