As another said earlier in the thread the JW’s splintered off the “Adventists” not necessarily SDA’s. Here is a short break-down of it.
A Baptist Preacher named William Miller predicted the definite timing of Christ’s 2nd Coming & in doing so attracted a large following of Christians from “many” different religions.
Eventually Mr. Miller’s teachings were proven to be false teachings when Jesus didn’t show up ( for the 2nd time ).
The second date provided for Jesus to arrive was provided by George Storrs ( a proto Jehovah’s Witness if you will ).
After it was realized that everything about predicting the definite timing of the 2nd Coming was bunk ( these people should have known this from Jesus’ own teaching on the subject ). The “Millerites” disbanded and returned to their former Churches leaving ONLY the anti-Trinitarians.
Soon these anti-Trinitarians started to squabble over what were and were not true teachings - they broke up into factions going slightly different directions. This is where we get the:
The above groups teach that Jesus was “subject to moral corruption” & as such yearned for sin but though prayer and dedication to His Father Jesus was able to resist these temptations and eventually become “Christ”.
The teaching goes on to say that “IF” Jesus would have sinned then God would have insured that Jesus rotted in the tomb and eternally become as if He never existed in the 1st place.
As if the above wasn’t bad enough Jesus is also claimed to actually be Michael the archangel.
Ellen White taught both the above points with militant, feverish velocity.