First run through I missed nearly all the notes and books in Quentin's lair because I thought how quickly I got there might make a difference on whether I saved mother or not, so scrabbling about on the floor looking at pieces of paper did not seem a top priority at the time. So when Orsino "confessed" at the end, it did seem more like it was something he had found out from Quentin's research after his belongings had been handed in for examination. Seemed sensible that the Templars would get the First Enchanter to examine stuff that they probably didn't fully understand, so I almost bought the excuse that he didn't want to tell Meredith because of the affect it would have.
Subsequent run throughs I made a point of collecting them all and that note clearly states "So glad to hear you've achieved the impossible", the impossible being able to create life out of death, and here is a further book to aid your research on the subject. The book was about necromancy and clearly used by Quentin. Orsino says later he thought it was too evil to use, so instead he sends it to his apostate friend. That in itself was unforgiveable. If he doesn't want it found on the premises, then burn the damn book, not send it out into the general populace and encourage the guy to use it by congratulating him on progress so far - I don't know, may be Quentin experimented initially with mice or cats before moving onto people. May be that's why Anders couldn't find any. And if you side with the Templars, with Bethany still alive and in the Circle, her reaction to the revelation that he was in contact with Quentin, confirms my own, "you could have stopped it". Even if he couldn't have stopped it without implicating himself, he sure as hell didn't have to assist it by supplying research information. Bear in mind that one of the mages from the Circle was the first to go missing and whilst Orsino might have assumed she just went apostate, he is still culpable in her murder as well as at least three other women.
Bethany, like my Hawke mage, was also utterly appalled that Orsino should have resorted to that magic no matter how desperate he felt. With the Templar ending it is more understandable as everyone seems to have abandoned him, but with the Mage ending, it was just a big fat betrayal whether I was a mage or not. At this point we are still keeping the mages alive and you never know, if we manage to take out Meredith, we may still salvage the situation and he does the one thing that will guarantee the Templars will show no mercy to any further mages that they find. I didn't potentially sacrifice the life of myself and my friends for this - we were meant to be going down fighting to protect innocent mages. If you want to give up, just go in a corner and self immolate, like the Qunari sarabas did, or better still self immolate while hugging Meredith.
One further point that strikes me about turning into an abomination - you promptly seem to lose all magical ability. It happens with the lesser transformations and it is true of the Harvester - just the ability to summon a few corpses and splatter some nasty draining liquid but that's it. If you are going to combine yourself into some uber monster, at least make sure that it endowes you with all the abilities of its individual parts - now that would be worthwhile - but just become some disgusting mindless blob - seriously! Also, how come abominations no longer explode after you kill them?