Rinaldo made the whip for his own use, to avenge his family, but couldn't do it because the whip was not perfect yet.
I suppose he used his knowlegde in alchemy to create a special weapon that could inflict more effective damage to vampires and other kind of creatures lurking in the castle, probably a whip because he doesn't look like the swordman type. Maybe he felt more comfortable with a long range weapon that allowed him to keep a certain distance from his enemies or just happened to have some training with that kind of weapons.
He says the whip suits Leon better, that's probably because Leon is a warrior and can use the whip better even if it's not his weapon of choice. The other weapons in his shop are probably normal, it can be that easy to create alchemic weapons and since he couldn't complete the whip why bother to make more weapons like that if he knows he can't acquire the missing ingredient, if Leon intended to defeat Walter he needed something better than a normal weapon and since he came empty handed Rinaldo gave him his old whip.