can someone help me understand?

Hi...first day posting, really need some help. Work with this jerk.

NC for just under 2 weeks so far. Left when I found out he was cheating (again!). Has always used the silent treatment to hurt me, along with gaslighting.

Ran out of his house in pain when I read texts on his phone that showed me he was indeed in some sort of a sexual situation with his roomate (read that it wasn't the first time, either, and he promised this had never and would not happen with her...she has herpes and now I am frantic about what my test resuls will say). He told me he was only going to watch his roomate have sex with another girl that he helped her pick up from a bar, and did not participate himself. I told him to never contact me again. He hasn't, I haven't either.

I have always gone running back and always contacted him regardless in the past, too repulsed this time. Sense he may be feeling this and is surprised. Anyway, I was devastated and he knew it, never expressed remorse. He told me he was going to end it anyway that day...he threatened this often, at least once a week, to shut up my suspicions. Anyway, so, he's been seeing me at work smiling, being OK (faking it, of course!) and even seeing me walking with another male after work. This would normally kill him. The following day, I was walking into work and crossed paths with him. Maintained NC, kept walking. He starting calling out to me, voice absolutely dripping with sarcasm, saying GOOD MORNING, how are YOU?!! and when I wouldn't turn around but kept walking he yelled, FINE! YOU have a GREAT DAY, then!! still dripping with sarcasm. I was embarrassed and felt stupid...I mean, what in the world was he mad at ME about??

Why would he do this? He now has it 'his' way and is out of the relationship, so he says...why would he need to be hurtful and try to engage me in some kind of talk? He already has the kind of sex he's been looking for, threesomes...so why bother with me? Why is he normally silent and now doing this??

He told me before I left that if I started anything at work, I would get it 'ten times over'. Nothing has happened on either side till now, and it's HIM that is doing it...I feel crazy right now, the craziness has not ended for me...I need to stop him from harassing me (probably won't be the last time, don't know??) but I have been barely surviving as it is. Think it is best that I tell him I will inform HR at work about his harassing me if he does not stop...he does not want to lose his job and doesn't want to be humiliated at work. I need to use this for my benefit I think to stop him from abusing me there...what do you think?

I understand NC and think it is best but I feel this is one instance that I am allowing him to walk all over me and it may be within my power (at work) to get him to stop. No one knows anything at work, but I have visited a counseling service at work (totally confidential) to talk about my feelings. BUT this was before the incident happened. So confused and any help and thoughts would be appreciated...thanks.