There's a lot of discussion about who can kill who, who can't kill who, and who can't kill themselves. There are all kinds of theories out there being stated as fact. I've been trying to remember back to what we know for sure, and there are really only 3 things that I can come up with that we know for sure:
1. MIB couldn't directly kill Jacob (he needed a loophole to do so, and found one)
2. Richard can't kill himself.
3. MIB can't kill the candidates.
There seem to be a lot of people taking it as fact that because Richard can't kill himself (he asked Jack to do it for him) that none of the candidates can kill themselves. Richard is not a candidate, and it seems in context of the story that Richard's inability to kill himself has more to do with the immortality that Jacob gave him. Just because Jacob touched the candidates and made them unable to be killed by MIB doesn't automatically mean that they can't commit suicide like Richard.
There are also posts suggesting that candidates can't kill other candidates, or that non-candidates can't kill candidates. Jack clearly said that MIB wanted them to kill each other because MIB himself could not kill them. It seems Jack is talking about candidates when he says this. As far as non-candidates not being able to kill candidates, I just don't see anything from the show that ever suggested that to be the case.
So, am I missing something? Were there other statements from the show about who can and can't kill who, besides those listed above? I've never missed an episode and I can't recall anything else that would lead me to believe that candidates can't commit suicide or kill each other.