The anger towards Thorn stems from the fact that it can basically one shot anyone that has taken damage. It leads to less skilled players being able to compete by camping certain part of the maps and simply putting one shot into someone to get a payoff kill. Generally, people who use it tend to use it as a ranged weapon, completely contrary it's class of weapon, and if it doesn't kill you, it basically leaves you in the brink of death and and easy scrap kill for anyone else.