Well you can't blame either of them, but more importantly Sarah you CAN NOT blame yourself. IT was going to happen sooner or later (as you said) but its unfortunate that it happened sooner rather than later.

All that matters is that he is ok! And I can assure you he does NOT hate his new family in the least bit. This was a rather unfortunate lesson, but it did teach him not to mess with Mandy, and this will probably never happen again

Cheer up and don't be sad! He is ok!