I'm not just pulling my assumptions out of thin air. They're based on his actions and the commentary I've listened to. No, I don't know him personally, but based on what I do know, I've formed an opinion, which I've shared.
I can admit that I don't know exactly what you base your opinion on (I haven't seen/heard/read many interviews, commentaries, videos with him), but what I reacted strongly to in your original post is that you assume he does things to piss people off, instead of creating poignant stories.
If you take the deaths/addictions away from the show, I think it wouldn't even be half as good as it is today. So for me; no pain, no gain. I assume it's the same for Joss.
It could be that he's sadistic, but I'm not ready to jump to that conclusion. Even if he was full of himself, I don't think he's more cruel than anyone else who tries to make the most out of a story.
Take Tara's death for example: Forst off, it was needed to bring Willow to a new place. He sacrificed one character so another one could grow. I might not like it, but that's because I would have prefered to see more of Tara than Willow at that point.
Second off; her time on the show, and with Willow, seemed the more lovely and bittersweet since he made sure we were so happy for Tara, before he took her away.
If it's not your cup of tea, it's not your cup of tea. But some off us might like storytwists like that, not because we like when people suffer, but we like to see the story move forward.
In the end, it's anyone's guess why Joss does what he does. I would just rather assume the best in people than the worst, when I don't have enough information(and yes, I understand that you're saying that you feel that you have enough info, which is of course up to you).
Whether he was going for daring or not, it came across as just cruel to me. How is what he did with Cordy, for instance, "daring" anyhow?
Exactly what do you mean? That he killed her, made her possessed, sleep with Connor, something else? I think her entire season 4 arc was daring. Not that entertaining, but not more cruel than the arcs I did find entertaining. Like killing Fred. I have never loved and admired her more than when she struggled for her life.
I feel like Cordelia's arc was a hit and a miss though. She had become boring, just there for Angel to have a shoulder to cry on, so something new was needed.
I don't know why Joss didn't write something that could make Cordelia return. I would guess that was because of the addition of Spike(I think the network might have demanded that), which kind of filled up the screen time. There might not have been any place for her. Maybe he couldn't come up with anything more for that character.
I was honestly shocked at how he treated Charisma, one of his longest working actors, because she got pregnant and didn't tell him. How insensitive! She'd had a miscarriage. I can completely understand wanting to wait to tell people to make sure you won't lose this one too. But pulling the rug from underneath her feet? Especially when she needed it the most, with a baby on the way?! That cemented his D-bag status to me.
Btw, why do you think he fired Charisma because he was mad about the baby and not because the story needed it? Or that she would need the money right then(I assume she had money in the bank because she was on a successful show, and maybe the father was there for her)? Do you have sources or is it a theory?