In The Vampire Diaries, we see a lot of supernatural things like vampires, werewolves, witches, doppelgängers and even hybrids. But what I don't get is why Klaus needs a doppelgängers blood to make hybrids and why he can't just use someone else's blood. And how come once the person is a vampire you can't use their blood anymore? They have to be human and a doppelgänger?
To break the curse, Klaus needed to sacrifice a Doppelgänger, but they tricked him and saved Elena.
Now Elena is the missing key to create a hybrid, that's why Klaus needed her blood. Katherine's blood can't be used, as she is a vampire now. That curse required to sacrifice one vampire, one werewolf and one Doppelgänger. Now adding Katherine to the mix will mess up the equation.
Why a vampire Doppelgänger can't be used was never made clear. Maybe when a Doppelgänger becomes a vampire his Doppelgänger status become nulled.
Klaus needs Elanas blood cause she is the doppelgänger and Klaus needs to sacrifice a doppelgänger to break the curse. He tried using Katharine's blood but she was able to get away and turn herself making her vampire and klaus can only use a doppelgängers blood as long as he/she is human. Her blood is also key to making more hybrids.