First of all, thank you for clarifying what you meant behind that message. And let me tell you my thought process when I was reading your post.
By their message I found their current state is pretty bad, because of the past, but I agree with you on this part, where we don't know how many people mistreated them (and what even exactly happened) and it could be even 100, but I thought this person should in the first place curse a person who mistreated them most, then if they would feel it is not enough, they would curse second person and so on, until they would feel it is time to stop it (and I personally think it wouldn't take that many people to feel that there was enough of revenge). And yes, handling many people or even few for a beginner can have negative effects, but doing it cautiously, for example by cleaning one's aura afterwards, they should be okay, and even if not, they would take bigger breaks between cursing next people.
When it comes what time in their life it happened at, I think it doesn't matter, because wrongdoing anytime is still wrongdoing, but definitely the longer bullying was lasting, more serious is case. And yes, being emotional can distort the perception of reality and I agree with you we don't know if it is a case or by what mental illness was caused if they have it.
If these bullys would regret what they did, it could help this person to feel a little bit better, but one may think it is not enough, because the damage was still done and they got away from it, so the revenge should be done.
To summarize, I agree with you on few points, especially when you were saying we don't have enough data and it would be the best when this person would give us details about it, so I realized stating this person should just do some cursing was too quick decision from me. The moment I was writing my previous message I just found that serious damage was done to that person, so proper revenge should take place. Also, right now I am not sure what should be done first: revenge or healing.