What is the relevancy of his ethnicity? 
His mixture of ethnicity's is very American common bringing in the curious "everyman" theme that is so quintessentially American.  This has intrigue for readers who, just as the experts, are trying to puzzle out just what drove this person to do what he did.  Astute readers are going to want to sift every clue available to them as their desire to correctly understand his motives, his personality drives their efforts.  To simply gloss over and erase out information to make him appear ethnically neutral would be both dishonest and a disservice to readers putting in some real effort to understand him. Trying to understand a serial killer, and the social fabric they navigate through is complex as it gets and any clues are appreciated.  

Is it relevant to mention he was of Italian and Filipino descent?  He was born in California which makes him American. 
His looks were such that he tended to blend in everywhere and anywhere, so yes, it is relevant.  
However, feel free to move that information deeper into the article.  
I don't understand how readers are supposed to draw the conclusion that being of Italian/ Filipino descent makes it easier to blend in.  The way the information is presented seems to form an impression that his ethnicity had something to do with his criminal actions.  My conclusion is that it is irrelevant and should be deleted. 
No, it made it easier for him to blend in in Miami.  
The following section, "Since he was of mixed race (his mother was white, and his father, Filipino), he blended in well in the diverse Miami Beach scene", all looks to me like original research.  Are there any references to support that this is a published conclusion?   
Just to clarify, I don't mean the part about his parents ethnicity in and of itself (that is documented), I'm referring to the idea that it had some special advantage which helped him fit in.  From my readings on this subject his ability to fit into the Miami scene was due to his gay lifestyle more than any other factor.