Being negative doesn't do anything but get you stressed.
Unless you mean something different than I do by "being negative"? By that I mean - being opposed to something can be a positive attitude. For instance, you can be opposed to sociopathic rent-seeking in a positive way.
Maybe..."don't be defeatist" is more specific than "don't be negative"? Don't sit there and crumble! Fight (hopefully nonviolently) back! Don't let the sociopaths win! You can't do that by relying on other sociopaths to do things for you. You have to get in the game, get motivated, act!
Also I think your analogy is flawed - grabbing someone with a knife to stop them from inflicting further harm is not "negative" - it's forceful perhaps, but it's a positive action. Negative is standing there going "oh they have a knife, it's terrible but we can't stop them, oh well I hope I'm not next." Positive is "maybe they'll get me, but I'm going to give 'em hell if they try!" Positive is definitely "Let's stop them so they can't do it again! If they happen to get me in the process, that's a sacrifice I'm willing to take, but there are more of us than them, let's get 'em!"
Which highlights a key: I posit that the way to combat these ills is with community, not with hyper-individualism, heros, or hoping "the right government" does it. Maybe perhaps "the right government" would work - but that's a type of what I mean by "the community" - a government is just a community after all...