I have found that if treated right, a servitor will act right. If you kick it around and treat it like a slave and it's a very thoughtful servitor, it isn't going to take your ignorant, arrogant behavior. And I personally think that anyone who treats a sentient being, whether they created it or not, with such disrespect deserves the same disrespect.
I've had a servitor "go wild", "become more powerful", "break off from me", etc, however he never attacked me, even though he began as a highly violent, highly destructive individual that I named Runihura, which means Destroyer. I never treated him or my lesser servitors as beneath me, and they never tried to hurt me. Runihura even changed himself into a wholly new form, but was obviously my creation.
As for the use of sigils, I've heard it's safer, but Runihura existed after the sigil I made on a stone rubbed off, but at this point, he was already free of me. I would also like to say, burning it can "lock" the sigil in place, as this was Spare's way to cause the sigil to happen as it was intended by using the sigil with the subconscious. Consider things carefully before you do them Klogg.
I concur with the use of energy working rather than a spell however.