THe story goes the witch killed the competition then buried silas, but do we know what happened afterwars?
If a normal human is killed with blood of a vampire in them, they return as vampires. Why would vampires make more of their kind and give their blood healing abilities, in this manner, unless it was part of the original immortal spell silas used. What if when the woman silas loved died, she was made into a vampire? The witch hid this fact from all, so silas would never find out. If she simply killed her agains, she would be trapped with her and silas forever. Instead, she was hidden away, dessicated or in stasis.
I had a theory that if silas is cured/killed, all vampire would be cured/killed. If silas is cured he is still a witch, so would be stuck in the veil, but as a mortal, the lover would die as a human, and be passed on completly. I had another theory that if stefan died permantly, it would shatter the main immortal spell with him as the doppleganger and having no one to carry on the bloodline, so killing silas, which means he and all vampires died. WHile she want him dead, she does not want silas and the woman he loves to ever get to gether.