My theory on this is, and really, I'm probably not the only one, is that this change in Jon is an side effort of Jon's resurrection. Perhaps this is a blessing of the lord of light. I hope Jon doesn't Theon kill but rather does what Ramsey did to him, keep him as a slave. Maybe even over time, Theon starts worshipping Jon as god, no man is capable of doing what he can.
But you know what would to turn this series from good to great, would be if Jon shared this power with someone else. Gendry, perhaps. Their fathers were friends. Or tormund, both brothers in arm.
Just throwing ideas out there, whatever direction you decide to go with I'll probably love.
Anyway can't wait to see what he does to euron.