Orsino supplies informationon Necromancy that he knows to be evil to a mage outside the circle system. What does he imagine he is going to use it for? That runs counter to the elf we meet at the end of Act 2, beginning of Act 3, who seems to be doing his best to operate within the law, protect his charges and has concern for ordinary citizens as well. He seemed to be one of the few powerful totally sane and sensible mages we encountered. In my head I could see my Hawke saying to his friend Fenris, see they're not all like in Tevinter and then after Orsino transforms, Fenris actually says effectively "told you so", to Bethany if not to myself.
The Orsino who sent the note was likely a much younger one, and possibly far more impulsive. He seems to have found a measure of restraint since then.
When we finally made it back out to Meredith having encountered possessed templars, blood mages, demons, skeletal undead, shades, arcane horrors and Orsino/Havester, I almost felt embarressed. Yes, I'd saved around a dozen normal mages, which was about the only thing a had to feel proud about but offset by a greater number of Templars who were probably pretty far down the pecking order and only doing their job. To be honest, it was a bit of relief she turned on me and I could get down to getting the memory of Orsino off my chest by undertaking a fight where didn't feel horribly let down, because she had always made it clear she would come after me for the choice I made. Whether I opted for Templars or Mages, I always guessed I'd have to take her on eventually.
Templars are soldiers. The purpose of a soldier is to fight and likely die. They made themselves the enemy.
The Orsino I thought I knew would have waited for Meredith and then let her have it in a full throttle magical assault. But he transforms when she is nowhere in sight, into a monstrosity that can't even do proper magic, and when the only people likely to suffer are his own allies, because we're going to die anyway? Thank you but I'd prefer to fall to my actual enemies - not the person I put my own neck on the line for. I tell you, my mage Hawke, had never felt more ashamed of being a mage and my other Hawkes want as little as possible to do with them in the future. So all in all, between them, Anders and Orsino have alienated a powerful potential ally for the cause - Score!
Well, the Harvester can summon lots of corpses and has a crapload of health, and is also not vulnerable to templar antimagic. As for the timing, he needed time and a lot of corpses; he could have used templar ones, but the armor might interfere with the binding. I think his plan was to charge out of the Gallows and leave a wide swathe for Hawke and company to follow in, but he couldn't control the form.