It is possible that Raublut was planning so far ahead, or he might just have made sure to befriend the prince, just in case it would become useful, or he might have had different plans initially, and then adapted to the new situation… or maybe he actually did feel warm toward the boy, but his devotion was so great he still used Hildebrand, despite their connection.

Considering how rather early on, Raublut encouraged Hildebrand to try to become the next Zent, I feel like Raublut having different plans and then adapting is the most likely option.