I find it kinda weird that some people here find torturing someone to death alright, but that is taking things too far.
If you think torturing someone to death after said person has been terrorizing the public is weird, then you must be living in a well-off place.
In Indonesia, we've had this rampant case of "begal", a robbery with most case being a "strike 1st then snatch". The perpetrators are on a motorcycle equipped with a varying length of bladed weapons, with some are about 30cm, but can even reach 1m.
When the people can get their hands on them, hoo boy we usually vent our frustration on them, and also sometimes burn them to death.
So the people's action in this manga is pretty normal.