I'm of the opinion that Seishirou did feel something for dear Subaru, though it's hard to know for sure just when it actually happened. Perhaps absense makes the heart grow fonder and he decided somewhere down the line that he was going to invoke the spell and thus be killed by the one he loves in one way or another.
Though I am getting off the topic here. At any rate, the only way I'll ever be convinced that Seishirou's last words were anything other than "I love you," is if CLAMP decides to flash back to that scene and actually tell us what that empty bubble actually said.
