Now can anyone tell me why that police officer in the video outright killed that man instead of simply shooting him in the leg?

It is because they are taught to solve these problems by kilingl.

If shooting is justified, shooting to INSTANTLY stop the threat is justified.

Shooting an arm or leg is FAR more likely to result in a miss, and guaranteed NOT to stop the person instantly.

He wasn't a cop but even cops aren't trained to take a chance in wounding vs. shooting to kill. This appears to be a situation of vigilantism rather than law enforcement officer excessive force. These guys were just civilians.

Studies have shown a person can cover 21 feet in less than a second. Someone under the influence of drugs or in a heightened emotional state can have a high pain tolerance, so hitting them in a non-vital area may not stop the attack. Or, the officer could miss because the leg is smaller and moving. If either of those things happen in your video, the officer is going to get stabbed. 

The intent of shooting is to stop the threat. The most efficient way to do that is the center of the chest. 

From that  recorded footage you provided  I would have to agree that the officer 👮 🚓🚔 used appropriate force to subdue the violent criminal stalker offender in such a time critical emergency - that psychotic brought the entire use of deadly force onto himself as even can't hear any audio the law enforcement agency officer 👮 gave him plenty of warnings summary to stay back or else and the officer was well within the parameters of the law to take action and use deadly force to stop the dangerous potentially fatal blow from that very long sharp blade being lunged by that psychotic assailant so basically there you have it - case closed even in my book though, there will be likely a internal affairs department investigation by the officers but, that recorded camera footage should clear that particular officer from any wrong doings and no charges will be likely laid as it looks to be a legally judicious for defence purposes only 

Conclusion : the attending officers or officer fulfilled all legal obligations before the use of such force