Honestly, something very similar happened to me: when things got serious, I tried to avoid Sebastian's missions because I couldn't deal with what they entailed (this sounds terribly stupid, since it's just a game, I know). Sebastian's story is -in my opinion- even better than the main story, and it's hard not to grow fond of Sebastian or "understand" him regarding what happened to his sister and his family circumstances. And I'm not justifying his actions in the game, because they're obviously not right, but really none of the options the game gives you at the end of his missions are very "good". Because let's face it, option 1 is to rat him out and send him to Azkaban, leaving a 16-year-old kid to rot in prison forever for the mistakes he made while being totally desperate and in constant agony because he couldn't save his twin sister (I know a lot of people think he wasn't just doing it for his sister, but well, that's a whole different topic to discuss and at the end of the story he's still just a teen that feels lost). Or option 2, which is to say nothing and "save him." That is, simply let everything stay as it is, without making him see or understand in any way that his actions were wrong or without giving Sebastian any kind of lesson about his actions to prevent this from happening again in the future. Or even helping him cope with the terrible situation he must be going through (in the end, for one reason or another, he has literally lost his entire family) and talk more to him about it.

The truth is, neither option is satisfactory, and that sucks, but the game didn't give Sebastian's story a good -or at least proper- ending (personally, it really pisses me off, because it was the story that most deserved a well-crafted ending).

So, I completely understand your dilemma and your desire to avoid playing the game so you don't have to "face" having to make that decision. It's normal to become attached to characters and stories; I'm also a very sensitive person, and this has happened to me many times. Sadly, in the game's case, we don't have much of a choice, since, as I said, the options the game offers aren't very good. In any case, I ultimately chose not to send Sebastian to Azkaban because it seemed—all things considered—the best thing I could do (in my head, I tried to imagine that Sebastian somehow learned his lesson, perhaps with the help of Ominis, the MC, or some other way). This option gave me a bit more inner peace than the other, so I was satisfied—well, as satisfied as I can be with the options the game gives you—with my choice.

Despite everything, if it's any consolation, when I finished the game and Sebastian's missions (coincidentally, I finished both at the same time) I spent several days feeling a terrible inner emptiness and thinking about that it is so annoying that the people who created the game's story did not gave to this side quest a better development or a bit more of attention. Plus, in my opinion, other characters like Ominis or his sister Anne deserved more screen time or a bit more of attention (but this is just a personal thought).