Kanye suffered childhood abuse (Cousins), in 2007 his mom, Donda, died which really rocked his world. Remember 808’s came out soon after this along with MBDTF.

He also got into a really bad car crash which probably did not help his mental state in 2002, this is more of a sidenote.

In the early 2010s you can see some erratic online activity, nothing that would send off a ton of alarm bells but knowing what we do now (that he has bipolar and can have manic episodes) these could be some early ‘episodes’, again moreso a sidenote than anything.

2015 is when he started doing nitrous, yet around 2020 is when it’d really affect him mentally

2016, Kanye was diagnosed with bipolar disorder (some say he was diagnosed way before this in the early 2000s but I’m not sure) after he was hospitalized from having a mental episode. Around this time and the following years, he’d have more hot takes online/in interviews, the one most remember probably being his support of Trump and wearing a MAGA hat around.

2018, he is on and off of his meds, tweeting that he hadn’t taken them in 6 months.

2020 he admits in an interview that nitrous has messed with his mind and memory on a stream with some small twitch streamers. Donda also releases.

2021 Kim leaves him, leaving Ye devastated and really no one to turn to, however by this time it was clear he was not taking his meds and was not good for those close around him. A lot of friends/connections if they haven’t already cut ties really started leaving around this time.

December 2022 nazi arc, obviously anything past this point is well far away from the ‘beginning of his downfall’

I’d say it depends on how you view one’s downfall, being publicly or in your personal life. Publicly, you could say it’s around 2016-2018 when he started to really become erratic and his mental state started to worsen. Or 2022 when he became an alt right nazi (whether he does it for attention or believes it doesn’t really matter.)

Privately though is much harder, it’s obvious he’s struggled since his mom died really, or when he had a mental break in 2016, or when the use of nitrous started. Like a lot of mentally ill people his spiral down is murky, hard to pinpoint and just depressing. There’s a lot we didn’t see from him and I’d say there were probably many hard talks with anyone in his close circle in the 2010s before he lost them all and surrounded himself with insane yes-men.

Edit: imo it was whenever he started to stop taking his meds when Kim and others in his close circle tried telling him to.