What is happening here?

[Howconvenient]

Can be delightful, funny, kind, likeable.
Then makes very inappropriate sexual banter.
Can be efficient, hard working, professional.
Then bangs on his desk and swears very aggressively in the office.
Clearly bad news.

But what is going on with me?
Why do I still occasionally find myself doubting what I have witnessed?
Why do I sometimes feel that maybe I am exaggerating the bad stuff, that I am bad mouthing a decent co-worker?
I keep having to check myself, whenever I enjoy a conversation with him, how do I not get a lot angrier at him for being so two faced?
I have met abusive and toxic people in the past . Haven't we all!
But never have I met anyone so confusing.
With hindsight the toxic people I have met were a lot more obvious.
This one can really charm anyone, then show a really sinister side of himself, and then I find myself wondering if it's all in my head?
Anyone experienced similar stuff?

But dont people who are arent nice and are abusive have the charming lovely side to them, as thats what draws people in ?

The charming, lovely side is often the learnt behaviour (the tactic) and the dark side is who they are. Good people don't behave like that.

Agreed. I know all the theory but to witness it is quite something else.
The worst part as I said is to still find myself on the verge of falling for the nice guy act. To wonder if I am the one with the issue, if I am over sensitive, etc.

Why are you doubting yourself? He is the one who has a problem.
I don't care how charming someone is, if they make inappropriate sexual comments and swear loudly in the office I'd steer well clear of them.

Part of the reason why I am doubting myself is because other people, who I would consider less naive than me , are even more charmed. Whenever I say something against him they don't want to hear it, even though they have heard the comments too. Others are less charmed, ie they don't like him particularly, but shrug him off as stupid but harmless.

I guess that's the thing. Sometimes I worry that he is not harmless, that something I seriously off. And then at other times I think I am being a drama queen. It is all very odd all the same.

To quote the old cliche - it's not you, it's him.

Just be thankful you aren't married to the arsehole.

Sounds like the bosses of Reynholm Industries: Denholm Reynholm (Chris Morris) and later, his son Douglas (Matt Berry) i.e. The I.T. Crowd.
Is he particularly talented at what he does? Thinking Gordon Ramsey or the boss (played by Meryl Streep) in The Devil wears Prada, sometimes truly gifted people seem to be allowed to behave inappropriately.

ContactLight yes definitely. I can totally see him being very controlling and jealous.
He is no more talented than the rest of us but he is good at his job yes.