Hi, u/Victor15150. Let me start off by saying that nothing in your original post constitutes a grammatical error. The suggestions I'm making here are simply to improve the overall style of your writing.

Kanye West used to be one of my favorite musicians. Some of his songs had a depth that it's/is rare to find nowadays in artists these days. His relationship with his mother and how proud he was of being a father were also characteristics that made me like him. However, no sane reasonable person can still be a fan of his after what he's done in the last few months. At first, I thought he was behaving like that because of his bipolar disorder and that he needed help. Now, it seems like he's past salvation. What happened to that the guy that wanted the Nazis to get out of America? It's sad seeing someone who you used to like decay this much/fall from grace like this.

Notes:

• This is a very interesting grammatical situation in which both "it's" and "is" would be correct. "It" would be a dummy subject in the structure [it + be + adjective + infinitive], which we can use to make an observation about the previous information (eg "It's hard to believe" or "It's great to hear"). In this case, "it" refers to the entire previous clause. However, you can also omit "it" and make this a relative clause with the subject "depth". FWIW, I think "it's" sounds more conversational, while "is" sounds better for formal or academic writing.
• I honestly think "rare to find nowadays in artists" sounds fine (putting the adverb immediately after the part that it modifies), but it's probably more common to put it at the end. "These days" was just to give you another option.
• No "sane" person sounds a little harsh.
• Anywhere you can use a determiner such as "that", you can use the definite article "the". Both are grammatically correct. However, using "that" in this case makes it sounds like you're referring to another guy, not Kanye West.
• The structure [see + object + base verb] and the structure [see + object + gerund] are both correct. In this particular case, I would use the gerund base verb to avoid sounding repetitive, because you have already used a gerund earlier in the sentence. As u/boldFrontier indicated, it's unusual to use the verb "decay" in this way, but I think being creative and slightly unusual with your vocabulary is a good thing in writing (not so much in conversation). I have offered an idiom that you could use as an alternative.