Regardless, I think we need to start making the internet a cleaner place for everybody.
That's just censorship by whoever gets to define "clean".
People need to act civilized online the same way they do at school or work.
No, because "online" isn't necessarily school or work, so there's no bureaucracy or autocracy to define "civilized" conduct.
If I'm your co-worker and I say I'll kill you and you know I'm truly mad at you, would you still feel like the matter doesn't require action?
Let's consider a possible example: "Goddamnit, I'm going to kill that son of a bitch; he broke the build 3 times this week". Nope, no action required.
This will reduce chances of action and will act as a deterrent to those wanting to make a dumb joke.
That last part is the problem. A law which has as an intended effect to deter those making dumb jokes (protected expression) is unconstitutional. A law which has an intended effect of something else but an unintended effect of deterring dumb jokes may be unconstitutional; to pass, the government would have to show that there was a compelling state interest in the "something else", and that the effect on protected speech was unavoidable and minimized as much as possible.