Why do you quit supporting him due to his party?
How did Jim's positions change?
What party a man is nominated by determines (A) who he will vote for and (B) who he will have to placate to govern and to get re-elected. Justice was a Republican who switched parties to avoid losing a primary and won many votes of folks who voted straight Republican except for him, knowing that Justice had been a lifelong Republican until 2015.
For Justice to get anything done, he would have had to switch to the GOP at some point. WV is solidly Republican at all levels, amazingly so. Justice is credible as a Republican, and he realized he would never get anything done as a Democrat.