You have raised multiple serious issues and should contact an attorney in your jurisdiction for more detailed advice, but generally in Allegheny County, either parent may file a complaint for custody using an attorney, a volunteer attorney or on their own.
If you are concerned about your child's welfare while in Father's care, you can file a custody complaint in conjunction with a Petition for Special Relief seeking supervised custody by Father. If you have already addressed custody through your divorce settlement, you could file a modification of custody and an emergency motion.
As far as his engaging in a repeated course of unwanted conduct, you may have grounds to file a criminal harassment complaint at your local MDJ. If you were personally threatened you are able to file for a PFA, which can include no-contact provisions as well as emergency custody relief. This can be done between 9 and 11 am at Family Court at 440 Ross Street, or after hours at your local MDJ or “Night Court” at 660 1st Avenue.
There are also 302 procedures which could commit him for a mental health evaluation, which you or a family member could pursue if you are seriously concerned about him hurting himself.
If you do not have the means for a paid lawyer you could contact Neighborhood Legal Services: http://www.nlsa.us/resources/family/family.html or the Allegheny County Lawyer Referral Service: http://www.acbalrs.org/, or the Women’s Center & Shelter of Greater Pittsburgh: Hotline (412) 687-8005.
The court will listen to your concerns in a custody matter. Note that you are unlikely to get "full" custody, but you may be awarded "primary" custody. The difference is that the Court will look for a way for your ex to be involved in your child's life, even if it is limited involvement.
You can obtain assistance to file for custody through the pro se motions program in Allegheny County, if you qualify financially. A link the pro se motions program is below.
This answer is for informative purposes only and should not be construed as legal advice. This Q&A forum does not create an attorney/client relationship. If you have a legal concern that is important to you, I urge you to consult with an attorney. For those who are concerned about their ability to afford legal assistance, the ACBA's Lawyer Referral Service is here to help: http://www.acbalrs.org .