There were various reports that Azov had been holding hostages in the theater before the bombing. These reports also state that they had rigged the building with explosives. The first casuality of war is the truth. We may never know what exactly happened. Even the wiki page cites estimated deaths from 12 - 600, quite a range there.
Here's a report from Max Blumenthal that includes the accounts of civilians that fled via RF humanitarian corridors. They all point to Azov acting as you might expect.
I remember VIVIDLY reading this tweet on r/GenZedong, then three days later randomly overhearing on the radio about "Russia's latest crime", bombing a theatre in Mariupol, and I thought THOSE NAZI MOTHERFUCKERS
It really is informative of how easily mainstream western media has willingly become a megaphone for neo-nazis.
It's not the first or last time, but once you comprehend a few glaring examples like this, you start to critically examine all the instances of propaganda that may have alluded your radar in the past.