Reading through, i think this is either smart decision maybe even given to him as advice to calm things down a bit while investigation goes or it's highly emotional one, which is very worrying to me.
If we start of emotional one, what this: "In the past one month, I have been criticized and hated by the people, and now all the domestic investigative agencies are investigating me, and I am being pushed back to the people. I can not tolerate myself to live alone and to avoid everyone around me. " and especially last sentence implies it's simply worrying. The thing is that no matter how strong you are as person, this hunt for him is nation-wide and not sth he can escape. It doesn't only affect him, but people close to him I have seen people attack other YG artist, attack Seungri's friends including Hara and FT.Island's members, attack his family members and so on. This can really take a toll on one's mental health and not everyone is able to take things like that, like everyone probably knows T.O.P and Daesung both got suicidal over their respective scandals and T.O.P even overdozing on benzos that was a likely suicide attempt (since that's most common way to attempt suicide, overdozing meds). So I really don't want to anything like that happen with Seungri even if he was guilty of the stuff he's being accused of. So that part of it is really worrying to me, the possibility of this really affecting his mental health and him making drastic decisions cause of that.
In other hand this might be sad, but rather smart way to get heat off him and calm things down a bit. If this is him taking time off from being celebirity, maybe influenced in this decision by YG, lawyers etc., the media might stop bombarding him that much and he can focus on investigation and enlistment with less noise going around. This might take lot of heat out of YG and YG artist as well letting them to return back in normal schedule, maybe on other BB members too. Then after the investigation and military service, if he's found innocent or innocent enough, he will be able to make a comeback either with BB or on his own, he does have connections and skills so there is lot of stuff he could do actually and he wouldn't even need to do it in South Korea- he could focus on his dj label, he could focus more on acting, he could focus being mc and on varities or he could simply be a businessman so it's not like he doesn't have future regardless of outcome i think. If this is a plan, there is chance people will call it out and don't let is succeed though. If it is I hope it does succeed since the current situation is not making favors to anyone other than being exhausting.
Still, whatever it is, I think the situation sucks. Seungri is someone who was been my ult bias since 2013 and he's someone i admire greatly. I cannot be anything, but devastated on what this whole situation has been and on his decision to do this.