(I checked US and Canadian laws and firing warning shots by police officers is VERY illegal and will land you in prison)

Well yeah, warning shots in the US are straight to the torso :P

2 Making clear and understandable verbal warning of the intention of the guard to use firearms, which may be accompanied by his legitimacy as an official performing his duties in security. Making a verbal warning in the opinion of some authors where not achieve the desired result (termination of the attack), it is necessary to duplicate production and a warning shot in a safe direction, which is defined as a final warning. As rightly stated in this connection, it practically is not always possible with a view specific conditions in individual cases. Such a warning is justified only in terms of what can be achieved respectable striker and demonstrate determination to use firearms to stop the attack, in addition, allow time for the attacker to execute warning to abandon the assault or unlawful behavior and obeyed his warning to the request;

That's for armed guards, I imagine police officers on duty follow the same protocol. Now, technically speaking you could read this in 2 ways - he fucked up by shooting in the air as a final warning. However, another interpretation would be - he could've simply shot straight to the perpetrators since he was in a situation where a warning shot could have adverse effects.

Many people in Bulgaria blamed him for shooting up knowingly he's under the bridge. I think that considering the circumstances, he took the more humane approach. Again, he was perfectly in his right to shoot straight at them in this situation. It's sad that the bullet bounced off the wall and killed a person but his actions were the best possible scenario if the information we've been given is accurate.