Reasons their actions were justified:
#1: Man (appearing unstable) is advancing towards them with no intention to stop with a DEADLY WEAPON
#2: Given the incidents that have been occurring recently with law enforcement, and the fact that Ferguson is only 20 minutes away from St. Louis, they had good reason to be fearful. This man could have wanted retribution against police for what he might have thought were hate crimes, wanting to punish these cops or cause them harm.
If you were in their shoes, given recent events, and a man was coming at you with a knife, I think you would act in self defense as well. It's not an easy situation to be in, and they made the call they thought best to protect themselves and others. Also, I think taser/pepper spray is used more in situations where the assailant is unarmed, but dangerous. This man was armed, and was dealt with accordingly. Not to say that this isn't very sad, and I wish things like this wouldn't happen, but I think this is one of those times where you cannot blame the police.