Ansel understood better than anyone the most terrifying aspect of fate.
The world generates infinite possibilities with each passing moment. Fate adheres to the logical progression of all things, yet can diverge from one reasonable outcome to create another plausible alternative.
When countless rational possibilities required by fate accumulate, it can ensure everything unfolds logically while simultaneously bending all to its will.
In his struggle against fate, Ansel could never hope to master the rationality of all things as fate does. His sole, most effective strategy was to narrow the rationality of involved elements to an absolutely favorable, unshakeable thread.
To successfully tame Seraphina, he had to eliminate all other possibilities that might sway her - remove Marlina, her family, her friends, everything she depended on, leaving only himself in her life.