Assuming Shinzon was telling the truth about his treatment by the Romulans, the psycological effects of that have almost certainly made him (partly) insane, which would allow for some inconsistency of character, and illogical decisions.
WHy the hatred for Picard? - jealousy, that Picard had, in Shinzon's opinion, had a good and easy life (maybe JLP should have told Shinzon about his experinece with the Borg), and also knowing that, if his plan to cripple the Federation was undertaken, leaving JLP alive would be dangerous, as Shinzon would be hunted down until one or other died.
As to not using the Scimitar against Romulus?, he didnt want to just destroy the Romulans, he probably wanted them to feel what he had been through. Start an all out war with the Federation, thereby ensuring the destruction of much of the old guard Romulan fleet, and taking over the RSE completely with a Reman command and military. This would then allow complete subjugation of the Romulan people, probably in preparation to turn them all into slaves, spending their entire lives suffering, rather than just being killed off relatively quickly. Also, Remus looks a barren world, with little liklihood of food production, so to use thalaron on Romulus would make supply and logistics more difficult.
and a small side point, I wonder if the Federation ever found a use for the single Scorpion in their shuttlebay after it was beamed on board the E? Unless there was a deleted scene indicating otherwise, I got the impression the whole ship was beamed aboard, not just Data and Picard.