If you can pull off the passive aggressive approach, you can nudge him into doing something really stupid (from the looks of it, it wouldn't be too hard) and getting him for that (legally). Don't bother doing this unless you're 100% confident because it could backfire depending on what happens/how you pull it off ;x.
My only advice is absolutely do not go rough him up. I know you want to, because if I had a sister who was being threatened I would as well, but in the long run you can fuck him over harder if you don't do that.
With that being said, keep an eye on your sister. If he goes there un-announced and threatens her, then feel free to fuck him up, lol
edit: also don't listen to these guys saying try to get him to do something stupid. You don't know how stupid he will get, and if he's actually a terrible person like you described you never know what he might do.
I dunno, what does "threaten" mean? Could it be he just wants to see his kid as a father? I agree with talking to the police though, but I would try to speak to somebody who can judge the situation more neutrally.
Lets not pretend we live in a world where people don't fight with concealed knives. It isn't safe to fight a junkie. They often have nothing to lose.
That said, I was getting death threats a few years ago until the person showed up at my work to threaten me again and I provoked him to hit me. I "injured" him, the police said. There was no problem after that.
And... THAT said... during the fight he grabbed a pen that was laying around and tried to stab me in the face with it, only cut open my scalp instead and luckily missed my eyes. He was a crazy son of a bitch.
I think the best bet might be getting her some dog spray for her purse or something stronger, some self defense training classes and a restraining order. Jiu jitsu is by far the best option if you want to invest in a few months.
If the threat's serious, don't let her go around by herself. Have her with a male escort at all times, somebody scary or scary looking.
Is talking to him and trying to understand his mindset and what he wants such a bad idea ? The more you know your ennemy the easier you can counter his actions..
He is probably a depressed piece of shit but whatever..
How old are you? Do you have a career? Do you realize that a criminal record can legitimately jeopardize your future employment options?
Lets say you or you with friends (taking friends to beat someone up? umm...) go and "teach him a lesson". You either do not do a very good job and nothing is gained or you do and we (Paramedics) are called. He is transported to the Hospital, reports that he was assaulted, Police arrive to take a statement and why wouldn't he press charges on you? What has he to lose? From the picture you've painted of him, nothing. What have you got to lose? I would think a lot more than nothing.
Police are there for a reason. The legal system works and the people who claim it doesn't are the people who have never used it.
If he seriously wants access to the child, then sadly there is nothing you can do. Maybe he hasn't been around for four years, he still has rights to access and they would be enforced if taken to court unless there is evidence (more than you saying hes a bad guy) like police records of abuse that would prevent him from seeing the child.