They killed Jim to isolate Tabitha
The story is building up to a situation where the entire town turns against her and Jade.
Jim, of course, would disapprove of the Tabitha X Jade pairing. come on.
But at the same time he also wouldn't turn against her, because despite their shaky marriage he still loves her.
Tabitha could never be fully isolated with Jim around.
The two narrative choices are
Jim betrays her (never happening)
Take Jim out of the picture.
The writers took option B. So Jim had to go.