By whom, the people that executed him? Did he actually stab someone else? If not, then where did that report come from?

By whom is completely irrelevant, whether it was already suspected by the cops or an anonymous caller who told them. Whether or not your innocent if you're a suspect your best interest is just to comply for the time being, specially if you look suspicious by openly carrying a knife around.

Yeah, because those are the two choices, summary execution or simply walking away free.

I really hope you aren't this dumb... Yeah let the suspect of a stabbing walk away free, nothing bad can happen right? it's not like they will get a call hour later of another stabbing right? Seriously this is the worse decision of them all, Specially since this person had showed he had 0 respect for the law already, if he walked away nothing would of stopped him from just doing another stabbing.

Once again the cops gave him a TON of time to cooperate and already used multiple non-lethal methods, nothing was working and the person simply didn't seem to care. You have to be a complete idiot at this point to simply walk away from the cops in the first place.