I have custody of our daughter (she lives with me), and my ex-boyfriend (her father) has in the past talked about killing himself, and the ways he could do it, he's also broken things that he owns (such as his digital camera) out of anger... should I get a lawyer? I do not trust him alone with her, and he doesn't understand that... what should I do?

Have you gone to court over custody or did you just agree to letting him see her?
How old is she?

I agree to him seeing her with supervision..
And she's 18 months...

If he is seeing her with supervision that is what is important.
You really should go to court and have an official visitation order written up as well as a child support order. Make sure you tell the Judge you feel he needs to continue with the supervised visits. This way if he is just wanting her at his convenience you can say your next visitation is Saturday after next between 1 and 3.

Also suggest to the Judge that you feel he needs to get counseling for his temper.