China was the 5th out of 6th claimants to weaponize their holdings on the SCS. The last being Brunei who has nothing. China was merely responding, not initiating SCS weaponization (but mostly in response to US pivot to Asia and increased US basing in the area to contain China - same reason Russia took Crimea in response to NATO encroachment led by US). The difference being China reclaiming land with 2% of Chinese GDP will vastly outscope Vietnam, Malaysia, Philippines or Taiwan. The irony is US pushing Philippines for PCA case against China using a dispute system China opted out during her UNCLOS ratification while US not being signatory to UNCLOS. Or US not recognizing features of other claimants that was also invalidated by PCA ruling which applied uniformly to other parties in the SCS.
I'm not sure there's a big difference between claiming they're territorial US waters vs claiming that the US gets to decide who's waters they are. In both cases the US has sovereignty over those waters
China is clearly the "aggressor" in this situation because they are building military installations in an area that was previously not a militarized zone.