Do you think raising my voice at the management and lobby guards would make any difference? Should I make a scene? I'm seriously very angry that even the management is not helping now. What is my mom paying maintenance fee for, if there is a security breach, and management is not doing their job?

Long way to go for many here. You are the very few!
Sharp and mature!     

I beg to differ, I applaud you for your kindness and leniency but this has nothing to do with maturity or being sharp, if you ask me, I would say that being mature is realizing when NOT to be lenient to your aggressor. Case in point: The tenant had threaten to harm OP's mother and had given numerous verbal threats. I think they are way passed the point of sitting down and have a heart to heart conversation, even if it's possible, there's no intention of any reformation, what you are offering is called "Enabling" Thus encouraging them to repeat with other landlords and prolonging this type of behavior, human beings do not reform through slip ups and evasion, they learn it through consequences and rewards. Let's be honest, if OP approach them with your method, who's really being rewarded here? I say them. Now I understand that as landlords; resorting to fire vs fire is futile.. TO SOME EXTENT of course because depending how far you can go, you will prevail, because that's what happened to me. But since the tenancy agreement is ending soon I think it doesn't hurt to go with your approach. If OP wants my service for his own satisfaction he can PM me. Either way I hope OP and his mother are doing well and safe.