This one was actually quite successful as a propaganda move. I see Western media (AP etc) peddle this now, some reports like "Russia claims Ukrainians attacked prison" and some without the "Russia claims" part. Kremlin will do everything they can to create the impression that HIMARS is used "improperly", they're really afraid of it and perhaps hoping that US will slow delivery because of this propaganda.
Anyone who 100% believes either side has an agenda...its war, and lies and propaganda are used on both sides.
Once its over, winner will decide who was liar and who was telling the truth
Pretty much this. Nevertheless we can make educational guesses.
What we know, in support of RU side:
-Russian side went to great length to capture those PoW
-Azov "evacuation" had the most global attention and was overseen by Red Cross
-They have great PR value. Parading PoWs on cameras is a great moral booster
-PoWs have direct values when doing PoW exchanges. And we know that some were already used in a PoW exchange
-Azov PoWs are especially prized, they're so valuable internally that Russia wanted to pass resolution banning their exchange
-Dead PoWs have no value to RU. It would be counter productive for RU. It can be used as moral booster and incentivize UA fighters to fight to the end instead of surrendering
-8 RU prison guards were also wounded in the shelling
In support of UA side:
-It's their most (?) prized military unit
-Azov PoWs had priority over other regular military PoWs
Summary of RU side claims as i see it:
-Azov PoWs started to talk with investigators and UA attacked to shut them up.
Summary of UA side claims as i see it:
-RU kills PoWs to pressures UA to stop using HIMARS
-RU kills PoWs so UA stops fighting
-Because that's just what orcs do
Considering the value of Azov being alive to UA and RU sides, both claims sound ridiculous to the point of being laughable. So most likely this was an accidental attack. As these things go, like with MH17, one side will try to do everything to investigate and drum up as much attention as possible, and the other side has to option but to just blatantly deny everything and try to make it go away ASAP.