as Jeff was about to kill you, it seems like he cannot stab your back, as he's heart was racing and racing when he sees you from behind, so he just runs away back and his feelings for you

AFTER 10 MONTHS, YOU GOT INTO A CAR ACCIDENT

Slenderman saw you that you were all alone, he offered you to join he's crew, and you accepted it, when slenderman introduced you to the others Jeff looked at you suprise that you were there, he just stares at you while you seem to just stare back