I'm a landlord. But i'm in Scotland, laws and things vary from country to country.
1. Could the landlord kick him out of his place for these sorts of troubles?
It's difficult to legally kick out a tennant. Very difficult actually. It depends on the tenancy agreement, too, you need to look at the terms of that. The legal system is so screwed up that anything can happen if your sharp enough. In general, over here, some kind of crap needs to be going on for atleast a few months before you can turn up with police and tell them to leave.
2. Could we get out of our lease because of this obvious violent and ill tempered neighbor?
I would go with what Stuward said. You need to find out exactly what you signed. You also deffinitly need to report it. If there's no record of you reporting any previous incident, then there's no record of anyone doing anything that might be considered wrong.
3. Any other advice you feel like giving...
Speak to the landlord. If it were me, I would go talk to him myself. I only have one property that I rent out but all the neighbours have my mobile phone number. I know most of them personally. If there's a problem with my tenant, i'll know. If the tenant has a problem with one of the neighbours, I wouldn't hesitate in knocking their door and having a chat. I can tell you for sure though that i'm not like most landlords but, it's deffinitly worth a try.
Also, do what nygmen said. People like that are often on the back foot when you catch them, say hi and shake their hand. Ask them if they want to come in for a coffee/beer/whatever. Bring up the situation and say your friends didn't know any better but they do now, and if there's any other problems, just give you a shout. If he doesn't bite then forget it. Atleast you tried. Most neighbourly fueds are just silly.
Our next door neighbour is a real bitch. Sounds harsh but she really is. We had a BBQ and she complained about the noise, even though we invited her and it's probably the only noise we've made in 6 months. Because nothing happened (sort of!), she's been raging ever since. She tries to make loads of noise early in the morning and things like that to annoy us. Most people on the street dislike her for similar reasons. Everytime I see her I ask her how she's doing, hows the house, etc. I know she hates it, but it amuses me and it makes it easy to live with. I think the only reason it goes on still is because my g/f can't let it go. She's raging about the whole thing. I could care less. I brought her bins in for her the other day just to rub salt in the wound. Sometimes revenge is best served with a smile.