Anyways, those are my recommendations. Apologies if it seems preachy. I would focus more on the story ahead of your saintess, rather than her backstory. Backstory is good for the characters she meets along the way, or is simple enough to summarize and make MC relatable while also explaining how MC gets through his/her decision making process.
So, the real story would be how she avoids the fact that she has killed someone, or even just wounded someone in combat, or if the summoners are corrupt, how she escapes their clutches and what she does afterwards. Avoid deus ex machinas as much as possible, where someone or something comes out of nowhere to rescue her from the situation. You can have her meet a character like a knight who helps, but set up their relationship and a believable way for the knight to be able to help. Then, treat the organization/summoners like the grim reaper or the monkeys in those speedrunning games where you die if you lose time to traps and such because of bad jumps. What I mean by that is, believably have them closing in on heroine, even when she's certain she's gaining distance or spending too much time with an adventuring party. Then, you could even have one or all members of that party sell her out. Maybe they think they're just selling her for a little extra cash, but she ends up in the hands of the organization, or royals who want to "breed" her powers into their bloodline, or doctos that want to exploit her, or someone who thinks he can gain immortality. She's not truly a saint, though, so maybe she overcomes her past trauma and kills to escape.